On 05.02.24 20:53, Sean Christopherson wrote:
> On Sat, Feb 03, 2024, Mathias Krause wrote:
>> Take 'dr6' from the arch part directly as already done for 'dr7'.
>> There's no need to take the clunky route via kvm_get_dr().
>>
>> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
>> ---
>> arch/x86/kvm/x86.c | 5 +----
>> 1 file changed, 1 insertion(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 13ec948f3241..0f958dcf8458 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5504,12 +5504,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
>> static void kvm_vcpu_ioctl_x86_get_debugregs(struct kvm_vcpu *vcpu,
>> struct kvm_debugregs *dbgregs)
>> {
>> - unsigned long val;
>> -
>> memset(dbgregs, 0, sizeof(*dbgregs));
>> memcpy(dbgregs->db, vcpu->arch.db, sizeof(vcpu->arch.db));
>> - kvm_get_dr(vcpu, 6, &val);
>> - dbgregs->dr6 = val;
>> + dbgregs->dr6 = vcpu->arch.dr6;
>
> Blech, kvm_get_dr() is so dumb, it takes an out parameter despite have a void
> return.
Jepp, that's why I tried to get rid of it.
> I would rather fix that wart and go the other direction, i.e. make dr7
> go through kvm_get_dr(). This obviously isn't a fast path, so the extra CALL+RET
> is a non-issue.
Okay. I thought, as this is an indirect call which is slightly more
expensive under RETPOLINE, I'd go the other way and simply open-code the
access, as done a few lines below in kvm_vcpu_ioctl_x86_set_debugregs().
But I don't mind that hard. You just mentioned last year[1], this part
shouldn't be squashed into what became patch 3 in this series.
[1] https://lore.kernel.org/kvm/ZCxarzBknX6o7dcb@xxxxxxxxxx/
> And if we wanted to fix that, e.g. for other paths that are
> slightly less slow, we should do so for all reads (and writes) to dr6 and dr7,
> e.g. provide dedicated APIs like we do for GPRs.
>
> Alternatively, I would probably be ok just open coding all direct reads and writes
> to dr6 and dr7. IIRC, at one point KVM was doing something meaningful in kvm_get_dr()
> for DR7 (which probably contributed to the weird API), but that's no longer the
> case.
Yeah, that special handling got simplified in commit 5679b803e44e ("KVM:
SVM: keep DR6 synchronized with vcpu->arch.dr6"). And yes, open-coding
the accesses would be my preferred option, as it's easier to read and
generates even less code. No need to have this indirection for a simple
member access.
>
> Actually, it probably makes sense to do both, i.e. do the below, and then open
> code all direct accesses. I think the odds of us needing wrappers around reading
> and writing guest DR6 and DR7 are quite low and there are enough existing open coded
> accesses that forcing them to convert would be awkward.
>
> I'll send a small two patch series.
>
> ---
> Subject: [PATCH] KVM: x86: Make kvm_get_dr() return a value, not use an out
> parameter
>
> TODO: writeme
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 2 +-
> arch/x86/kvm/emulate.c | 17 ++++-------------
> arch/x86/kvm/kvm_emulate.h | 2 +-
> arch/x86/kvm/smm.c | 15 ++++-----------
> arch/x86/kvm/svm/svm.c | 7 ++-----
> arch/x86/kvm/vmx/nested.c | 2 +-
> arch/x86/kvm/vmx/vmx.c | 5 +----
> arch/x86/kvm/x86.c | 20 ++++++++------------
> 8 files changed, 22 insertions(+), 48 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ad5319a503f0..464fa2197748 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2046,7 +2046,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3);
> int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4);
> int kvm_set_cr8(struct kvm_vcpu *vcpu, unsigned long cr8);
> int kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val);
> -void kvm_get_dr(struct kvm_vcpu *vcpu, int dr, unsigned long *val);
> +unsigned long kvm_get_dr(struct kvm_vcpu *vcpu, int dr);
> unsigned long kvm_get_cr8(struct kvm_vcpu *vcpu);
> void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw);
> int kvm_emulate_xsetbv(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 695ab5b6055c..33444627fcf4 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3011,7 +3011,7 @@ static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
> ret = em_push(ctxt);
> }
>
> - ops->get_dr(ctxt, 7, &dr7);
> + dr7 = ops->get_dr(ctxt, 7);
> ops->set_dr(ctxt, 7, dr7 & ~(DR_LOCAL_ENABLE_MASK | DR_LOCAL_SLOWDOWN));
>
> return ret;
> @@ -3866,15 +3866,6 @@ static int check_cr_access(struct x86_emulate_ctxt *ctxt)
> return X86EMUL_CONTINUE;
> }
>
> -static int check_dr7_gd(struct x86_emulate_ctxt *ctxt)
> -{
> - unsigned long dr7;
> -
> - ctxt->ops->get_dr(ctxt, 7, &dr7);
> -
> - return dr7 & DR7_GD;
> -}
> -
> static int check_dr_read(struct x86_emulate_ctxt *ctxt)
> {
> int dr = ctxt->modrm_reg;
> @@ -3887,10 +3878,10 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
> if ((cr4 & X86_CR4_DE) && (dr == 4 || dr == 5))
> return emulate_ud(ctxt);
>
> - if (check_dr7_gd(ctxt)) {
> + if (ctxt->ops->get_dr(ctxt, 7) & DR7_GD) {
> ulong dr6;
>
> - ctxt->ops->get_dr(ctxt, 6, &dr6);
> + dr6 = ctxt->ops->get_dr(ctxt, 6);
> dr6 &= ~DR_TRAP_BITS;
> dr6 |= DR6_BD | DR6_ACTIVE_LOW;
> ctxt->ops->set_dr(ctxt, 6, dr6);
> @@ -5449,7 +5440,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
> ctxt->dst.val = ops->get_cr(ctxt, ctxt->modrm_reg);
> break;
> case 0x21: /* mov from dr to reg */
> - ops->get_dr(ctxt, ctxt->modrm_reg, &ctxt->dst.val);
> + ctxt->dst.val = ops->get_dr(ctxt, ctxt->modrm_reg);
> break;
> case 0x40 ... 0x4f: /* cmov */
> if (test_cc(ctxt->b, ctxt->eflags))
> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> index 4351149484fb..5382646162a3 100644
> --- a/arch/x86/kvm/kvm_emulate.h
> +++ b/arch/x86/kvm/kvm_emulate.h
> @@ -203,7 +203,7 @@ struct x86_emulate_ops {
> ulong (*get_cr)(struct x86_emulate_ctxt *ctxt, int cr);
> int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val);
> int (*cpl)(struct x86_emulate_ctxt *ctxt);
> - void (*get_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong *dest);
> + ulong (*get_dr)(struct x86_emulate_ctxt *ctxt, int dr);
> int (*set_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong value);
> int (*set_msr_with_filter)(struct x86_emulate_ctxt *ctxt, u32 msr_index, u64 data);
> int (*get_msr_with_filter)(struct x86_emulate_ctxt *ctxt, u32 msr_index, u64 *pdata);
> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
> index dc3d95fdca7d..f5a30d3a44a1 100644
> --- a/arch/x86/kvm/smm.c
> +++ b/arch/x86/kvm/smm.c
> @@ -184,7 +184,6 @@ static void enter_smm_save_state_32(struct kvm_vcpu *vcpu,
> struct kvm_smram_state_32 *smram)
> {
> struct desc_ptr dt;
> - unsigned long val;
> int i;
>
> smram->cr0 = kvm_read_cr0(vcpu);
> @@ -195,10 +194,8 @@ static void enter_smm_save_state_32(struct kvm_vcpu *vcpu,
> for (i = 0; i < 8; i++)
> smram->gprs[i] = kvm_register_read_raw(vcpu, i);
>
> - kvm_get_dr(vcpu, 6, &val);
> - smram->dr6 = (u32)val;
> - kvm_get_dr(vcpu, 7, &val);
> - smram->dr7 = (u32)val;
> + smram->dr6 = (u32)kvm_get_dr(vcpu, 6);
> + smram->dr7 = (u32)kvm_get_dr(vcpu, 7);
>
> enter_smm_save_seg_32(vcpu, &smram->tr, &smram->tr_sel, VCPU_SREG_TR);
> enter_smm_save_seg_32(vcpu, &smram->ldtr, &smram->ldtr_sel, VCPU_SREG_LDTR);
> @@ -231,7 +228,6 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
> struct kvm_smram_state_64 *smram)
> {
> struct desc_ptr dt;
> - unsigned long val;
> int i;
>
> for (i = 0; i < 16; i++)
> @@ -240,11 +236,8 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
> smram->rip = kvm_rip_read(vcpu);
> smram->rflags = kvm_get_rflags(vcpu);
>
> -
> - kvm_get_dr(vcpu, 6, &val);
> - smram->dr6 = val;
> - kvm_get_dr(vcpu, 7, &val);
> - smram->dr7 = val;
> + smram->dr6 = kvm_get_dr(vcpu, 6);
> + smram->dr7 = kvm_get_dr(vcpu, 7);;
>
> smram->cr0 = kvm_read_cr0(vcpu);
> smram->cr3 = kvm_read_cr3(vcpu);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e90b429c84f1..dda91f7cd71b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2735,7 +2735,6 @@ static int dr_interception(struct kvm_vcpu *vcpu)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> int reg, dr;
> - unsigned long val;
> int err = 0;
>
> /*
> @@ -2763,11 +2762,9 @@ static int dr_interception(struct kvm_vcpu *vcpu)
> dr = svm->vmcb->control.exit_code - SVM_EXIT_READ_DR0;
> if (dr >= 16) { /* mov to DRn */
> dr -= 16;
> - val = kvm_register_read(vcpu, reg);
> - err = kvm_set_dr(vcpu, dr, val);
> + err = kvm_set_dr(vcpu, dr, kvm_register_read(vcpu, reg));
> } else {
> - kvm_get_dr(vcpu, dr, &val);
> - kvm_register_write(vcpu, reg, val);
> + kvm_register_write(vcpu, reg, kvm_get_dr(vcpu, dr));
> }
>
> return kvm_complete_insn_gp(vcpu, err);
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 994e014f8a50..28d1088a1770 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4433,7 +4433,7 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
> (vm_entry_controls_get(to_vmx(vcpu)) & VM_ENTRY_IA32E_MODE);
>
> if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_DEBUG_CONTROLS)
> - kvm_get_dr(vcpu, 7, (unsigned long *)&vmcs12->guest_dr7);
> + vmcs12->guest_dr7 = kvm_get_dr(vcpu, 7);
>
> if (vmcs12->vm_exit_controls & VM_EXIT_SAVE_IA32_EFER)
> vmcs12->guest_ia32_efer = vcpu->arch.efer;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e262bc2ba4e5..aa47433d0c9b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5566,10 +5566,7 @@ static int handle_dr(struct kvm_vcpu *vcpu)
>
> reg = DEBUG_REG_ACCESS_REG(exit_qualification);
> if (exit_qualification & TYPE_MOV_FROM_DR) {
> - unsigned long val;
> -
> - kvm_get_dr(vcpu, dr, &val);
> - kvm_register_write(vcpu, reg, val);
> + kvm_register_write(vcpu, reg, kvm_get_dr(vcpu, dr));
> err = 0;
> } else {
> err = kvm_set_dr(vcpu, dr, kvm_register_read(vcpu, reg));
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c339d9f95b4b..b2357009bbbe 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1399,21 +1399,21 @@ int kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
> }
> EXPORT_SYMBOL_GPL(kvm_set_dr);
>
> -void kvm_get_dr(struct kvm_vcpu *vcpu, int dr, unsigned long *val)
> +unsigned long kvm_get_dr(struct kvm_vcpu *vcpu, int dr)
> {
> size_t size = ARRAY_SIZE(vcpu->arch.db);
>
> switch (dr) {
> case 0 ... 3:
> - *val = vcpu->arch.db[array_index_nospec(dr, size)];
> + return vcpu->arch.db[array_index_nospec(dr, size)];
> break;
> case 4:
> case 6:
> - *val = vcpu->arch.dr6;
> + return vcpu->arch.dr6;
> break;
> case 5:
> default: /* 7 */
> - *val = vcpu->arch.dr7;
> + return vcpu->arch.dr7;
> break;
> }
> }
> @@ -5510,13 +5510,10 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
> static void kvm_vcpu_ioctl_x86_get_debugregs(struct kvm_vcpu *vcpu,
> struct kvm_debugregs *dbgregs)
> {
> - unsigned long val;
> -
> memset(dbgregs, 0, sizeof(*dbgregs));
> memcpy(dbgregs->db, vcpu->arch.db, sizeof(vcpu->arch.db));
> - kvm_get_dr(vcpu, 6, &val);
> - dbgregs->dr6 = val;
> - dbgregs->dr7 = vcpu->arch.dr7;
> + dbgregs->dr6 = kvm_get_dr(vcpu, 6);
> + dbgregs->dr7 = kvm_get_dr(vcpu, 7);
> }
>
> static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,
> @@ -8165,10 +8162,9 @@ static void emulator_wbinvd(struct x86_emulate_ctxt *ctxt)
> kvm_emulate_wbinvd_noskip(emul_to_vcpu(ctxt));
> }
>
> -static void emulator_get_dr(struct x86_emulate_ctxt *ctxt, int dr,
> - unsigned long *dest)
> +static unsigned long emulator_get_dr(struct x86_emulate_ctxt *ctxt, int dr)
> {
> - kvm_get_dr(emul_to_vcpu(ctxt), dr, dest);
> + return kvm_get_dr(emul_to_vcpu(ctxt), dr);
> }
>
> static int emulator_set_dr(struct x86_emulate_ctxt *ctxt, int dr,
>
> base-commit: 60eedcfceda9db46f1b333e5e1aa9359793f04fb
As this provides a saner API to kvm_set_dr(),
Acked-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
