On Tue, Jan 14, 2020 at 08:25:40PM -0500, Krish Sadhukhan wrote: > According to section "Checks on Guest Control Registers, Debug Registers, and > and MSRs" in Intel SDM vol 3C, the following checks are performed on vmentry > of nested guests: > > If the "load debug controls" VM-entry control is 1, bits 63:32 in the DR7 > field must be 0. Please explain *why* the check is being added to KVM. Quoting the SDM is very helpful in proving the correctness of the code, but it doesn't provide any insight into why a guest field is being checked in software. A tweaked version of Jim's anaylsis from v1[*] would be perfect. https://lkml.kernel.org/r/CALMp9eR2GQ_aerH-arOEpa08k8ZdtYCA5ftxHfDCo5fS1r3VtA@xxxxxxxxxxxxxx > Signed-off-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx> > Reviewed-by: Karl Heubaum <karl.heubaum@xxxxxxxxxx> > --- > arch/x86/kvm/vmx/nested.c | 6 ++++++ > arch/x86/kvm/x86.c | 2 +- > arch/x86/kvm/x86.h | 6 ++++++ > 3 files changed, 13 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index 4aea7d304beb..acde8a2f13e2 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -2899,6 +2899,12 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu, > CC(!nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4))) > return -EINVAL; > > +#ifdef CONFIG_X86_64 Hmm, I'd prefer not to wrap this with CONFIG_X86_64. From an architectural perspective, the consistency check is performed if the CPU *supports* long mode, irrespective of whether the CPU is actually in long mode. KVM could technically do something like static_cpu_has(X86_FEATURE_LM), but that's a waste of code for everyone except the 0.00000000000001% of the population running on Yonah, and nested 32-bit on 64-bit already fudges things with respect to 64-bit CPU behavior. Functionally, it'll be the same end result (and possibly a waste of cycles on 32-bit KVM if the compiler doesn't optimize out kvm_dr7_valid()) as having the CONFIG_X86_64 since kvm_dr7_valid() will always return true on 32-bit KVM (assuming @data is changed to an unsigned long). Architecturally, 32-bit KVM on 64-bit harware is already in a grey area, e.g. hardware VM-Entry still performs checks like GUEST_DR7[63:32]!=0, they just can't fail on 32-bit KVM because KVM's VMWRITE to propgate vmcs12->guest_dr7 to vmcs02.GUEST_DR7 will drop bits 63:32. In other words, it's not an issue of functionality, I'd just prefer to keep keep the constency checks themselves aligned with the SDM. > + if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS) && > + !kvm_dr7_valid(vmcs12->guest_dr7)) Wrap !kvm_dr7_valid() with CC() so that it's traced. > + return -EINVAL; > +#endif > + > if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PAT) && > CC(!kvm_pat_valid(vmcs12->guest_ia32_pat))) > return -EINVAL; > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index cf917139de6b..220f20a2f9c3 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -1064,7 +1064,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val) > case 5: > /* fall through */ > default: /* 7 */ > - if (val & 0xffffffff00000000ULL) > + if (!kvm_dr7_valid(val)) > return -1; /* #GP */ > vcpu->arch.dr7 = (val & DR7_VOLATILE) | DR7_FIXED_1; > kvm_update_dr7(vcpu); > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > index 29391af8871d..76cd389ecf60 100644 > --- a/arch/x86/kvm/x86.h > +++ b/arch/x86/kvm/x86.h > @@ -369,6 +369,12 @@ static inline bool kvm_pat_valid(u64 data) > return (data | ((data & 0x0202020202020202ull) << 1)) == data; > } > > +static inline bool kvm_dr7_valid(u64 data) Per Jim's feedback on v1, @data should be "unsigned long". > +{ > + /* Bits [63:32] are reserved */ > + return ((data & 0xFFFFFFFF00000000ull) ? false : true); Per Jim's feedback in v1, the ternary operator and second set of parantheses are unnecessary. return !(data & 0xFFFFFFFF00000000ull); or return data == (u32)data; or return !(data >> 32); I prefer the last one because (IMO) it's easier to visually parse than the "& 0xFF..." variant and more explicit in what it's doing than the casting variant. > +} > + > void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu); > void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu); > > -- > 2.20.1 >
