Currently, host_rsp is cached on a per-vCPU basis, i.e. it's stored in
struct vcpu_vmx. In non-nested usage the caching is for all intents
and purposes 100% effective, e.g. only the first VMLAUNCH needs to
synchronize VMCS.HOST_RSP since the call stack to vmx_vcpu_run() is
identical each and every time. But when running a nested guest, KVM
must invalidate the cache when switching the current VMCS as it can't
guarantee the new VMCS has the same HOST_RSP as the previous VMCS. In
other words, the cache loses almost all of its efficacy when running a
nested VM.
Move host_rsp to struct vmcs_host_state, which is per-VMCS, so that it
is cached on a per-VMCS basis and restores its 100% hit rate when
nested VMs are in play.
Note that the host_rsp cache for vmcs02 essentially "breaks" when
nested early checks are enabled as nested_vmx_check_vmentry_hw() will
see a different RSP at the time of its VM-Enter. While it's possible
to avoid even that VMCS.HOST_RSP synchronization, e.g. by employing a
dedicated VM-Exit stack, there is little motivation for doing so as
the overhead of two VMWRITEs (~55 cycles) is dwarfed by the overhead
of the extra VMX transition (600+ cycles) and is a proverbial drop in
the ocean relative to the total cost of a nested transtion (10s of
thousands of cycles).
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
arch/x86/kvm/vmx/nested.c | 28 +++++++++-------------------
arch/x86/kvm/vmx/vmcs.h | 1 +
arch/x86/kvm/vmx/vmx.c | 13 ++++++-------
arch/x86/kvm/vmx/vmx.h | 1 -
4 files changed, 16 insertions(+), 27 deletions(-)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 33235fc0a8fc..9be3156f972d 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1978,17 +1978,6 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs)
prepare_vmcs02_early_full(vmx, vmcs12);
- /*
- * HOST_RSP is normally set correctly in vmx_vcpu_run() just before
- * entry, but only if the current (host) sp changed from the value
- * we wrote last (vmx->host_rsp). This cache is no longer relevant
- * if we switch vmcs, and rather than hold a separate cache per vmcs,
- * here we just force the write to happen on entry. host_rsp will
- * also be written unconditionally by nested_vmx_check_vmentry_hw()
- * if we are doing early consistency checks via hardware.
- */
- vmx->host_rsp = 0;
-
/*
* PIN CONTROLS
*/
@@ -2755,8 +2744,12 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
asm(
/* Set HOST_RSP */
"sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */
- __ex("vmwrite %%" _ASM_SP ", %%" _ASM_DX) "\n\t"
- "mov %%" _ASM_SP ", %c[host_rsp](%1)\n\t"
+ "cmp %%" _ASM_SP ", (%% " _ASM_DI") \n\t"
+ "je 1f \n\t"
+ "mov %%" _ASM_SP ", (%% " _ASM_DI") \n\t"
+ "mov $%c[HOST_RSP], %%" _ASM_DI " \n\t"
+ __ex("vmwrite %%" _ASM_SP ", %%" _ASM_DI) "\n\t"
+ "1: \n\t"
"add $%c[wordsize], %%" _ASM_SP "\n\t" /* un-adjust RSP */
/* Check if vmlaunch or vmresume is needed */
@@ -2766,11 +2759,11 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
/* Set vmx->fail accordingly */
"setbe %c[fail](%% " _ASM_CX")\n\t"
- : ASM_CALL_CONSTRAINT
- : "c"(vmx), "d"((unsigned long)HOST_RSP),
+ : ASM_CALL_CONSTRAINT, "=D"((int){0})
+ : "c"(vmx), "D"(&vmx->loaded_vmcs->host_state.rsp),
+ [HOST_RSP]"i"(HOST_RSP),
[launched]"i"(offsetof(struct vcpu_vmx, __launched)),
[fail]"i"(offsetof(struct vcpu_vmx, fail)),
- [host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
[wordsize]"i"(sizeof(ulong))
: "rax", "cc", "memory"
);
@@ -3904,9 +3897,6 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
vmx_flush_tlb(vcpu, true);
}
- /* This is needed for same reason as it was needed in prepare_vmcs02 */
- vmx->host_rsp = 0;
-
/* Unpin physical memory we referred to in vmcs02 */
if (vmx->nested.apic_access_page) {
kvm_release_page_dirty(vmx->nested.apic_access_page);
diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 6def3ba88e3b..cb6079f8a227 100644
--- a/arch/x86/kvm/vmx/vmcs.h
+++ b/arch/x86/kvm/vmx/vmcs.h
@@ -34,6 +34,7 @@ struct vmcs_host_state {
unsigned long cr4; /* May not match real cr4 */
unsigned long gs_base;
unsigned long fs_base;
+ unsigned long rsp;
u16 fs_sel, gs_sel, ldt_sel;
#ifdef CONFIG_X86_64
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3f144a7fcfdb..3ecb4c86a240 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6127,9 +6127,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
"sub $%c[wordsize], %%" _ASM_SP "\n\t" /* placeholder for guest rcx */
"push %%" _ASM_CX " \n\t"
"sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */
- "cmp %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t"
+ "cmp %%" _ASM_SP ", (%%" _ASM_DI ") \n\t"
"je 1f \n\t"
- "mov %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t"
+ "mov %%" _ASM_SP ", (%%" _ASM_DI ") \n\t"
/* Avoid VMWRITE when Enlightened VMCS is in use */
"test %%" _ASM_SI ", %%" _ASM_SI " \n\t"
"jz 2f \n\t"
@@ -6221,11 +6221,10 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
"xor %%esi, %%esi \n\t"
"xor %%edi, %%edi \n\t"
"pop %%" _ASM_BP " \n\t"
- : ASM_CALL_CONSTRAINT, "=S"((int){0})
- : "c"(vmx), "S"(evmcs_rsp),
+ : ASM_CALL_CONSTRAINT, "=D"((int){0}), "=S"((int){0})
+ : "c"(vmx), "D"(&vmx->loaded_vmcs->host_state.rsp), "S"(evmcs_rsp),
[launched]"i"(offsetof(struct vcpu_vmx, __launched)),
[fail]"i"(offsetof(struct vcpu_vmx, fail)),
- [host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
[HOST_RSP]"i"(HOST_RSP),
[rax]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RAX])),
[rbx]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RBX])),
@@ -6248,10 +6247,10 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
[wordsize]"i"(sizeof(ulong))
: "cc", "memory"
#ifdef CONFIG_X86_64
- , "rax", "rbx", "rdx", "rdi"
+ , "rax", "rbx", "rdx"
, "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
#else
- , "eax", "ebx", "edx", "edi"
+ , "eax", "ebx", "edx"
#endif
);
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index ab15a905b71b..2138ddffb1cf 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -155,7 +155,6 @@ struct nested_vmx {
struct vcpu_vmx {
struct kvm_vcpu vcpu;
- unsigned long host_rsp;
u8 fail;
u8 msr_bitmap_mode;
u32 exit_intr_info;
--
2.19.2
