[PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

Yang Weijiang
2023-08-03T07:37:14Z

Update CPUID(EAX=0DH,ECX=1) when the guest's XSS is modified.
CPUID(EAX=0DH,ECX=1).EBX reports required storage size of
all enabled xstate features in XCR0 | XSS. Guest can allocate
sufficient xsave buffer based on the info.

Note, KVM does not yet support any XSS based features, i.e.
supported_xss is guaranteed to be zero at this time.

Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/cpuid.c | 20 ++++++++++++++++++--
arch/x86/kvm/x86.c | 8 +++++---
3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 28bd38303d70..20bbcd95511f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -804,6 +804,7 @@ struct kvm_vcpu_arch {

u64 xcr0;
u64 guest_supported_xcr0;
+ u64 guest_supported_xss;

struct kvm_pio_request pio;
void *pio_data;
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 7f4d13383cf2..0338316b827c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -249,6 +249,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
}

+static u64 cpuid_get_supported_xss(struct kvm_cpuid_entry2 *entries, int nent)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = cpuid_entry2_find(entries, nent, 0xd, 1);
+ if (!best)
+ return 0;
+
+ return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
+}
+
static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
int nent)
{
@@ -276,8 +287,11 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e

best = cpuid_entry2_find(entries, nent, 0xD, 1);
if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
- cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
- best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
+ cpuid_entry_has(best, X86_FEATURE_XSAVEC))) {
+ u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
+
+ best->ebx = xstate_required_size(xstate, true);
+ }

best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
if (kvm_hlt_in_guest(vcpu->kvm) && best &&
@@ -325,6 +339,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)

vcpu->arch.guest_supported_xcr0 =
cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
+ vcpu->arch.guest_supported_xss =
+ cpuid_get_supported_xss(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);

/*
* FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0b9033551d8c..5d6d6fa33e5b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3780,10 +3780,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
* IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
* XSAVES/XRSTORS to save/restore PT MSRs.
*/
- if (data & ~kvm_caps.supported_xss)
+ if (data & ~vcpu->arch.guest_supported_xss)
return 1;
- vcpu->arch.ia32_xss = data;
- kvm_update_cpuid_runtime(vcpu);
+ if (vcpu->arch.ia32_xss != data) {
+ vcpu->arch.ia32_xss = data;
+ kvm_update_cpuid_runtime(vcpu);
+ }
break;
case MSR_SMI_COUNT:
if (!msr_info->host_initiated)
--
2.27.0