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

Sean Christopherson
2023-08-04T16:02:43Z

On Thu, Aug 03, 2023, Yang Weijiang wrote:
> 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.

Please wrap changelogs closer to ~75 chars. I'm pretty sure this isn't the first
time I've made this request...

> 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;

Nit, the variable should be xfeatures, not xstate. Though I vote to avoid the
variable entirely,

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 |
vcpu->arch.ia32_xss, true);

though it's only a slight preference, i.e. feel free to keep your approach if
you or others feel strongly about the style.

> + }
>
> 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);

Blech. I tried to clean up this ugly, but Paolo disagreed[*]. Can you fold in
the below (compile tested only) patch at the very beginning of this series? It
implements my suggested alternative. And then this would become:

static u64 vcpu_get_supported_xss(struct kvm_vcpu *vcpu)
{
struct kvm_cpuid_entry2 *best;

best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
if (!best)
return 0;

return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
}

[*] https://lore.kernel.org/all/ZGfius5UkckpUyXl@google.com

> /*
> * 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);
> + }

Nit, I prefer this style:

if (vcpu->arch.ia32_xss == data)
break;

vcpu->arch.ia32_xss = data;
kvm_update_cpuid_runtime(vcpu);

so that the common path isn't buried in an if-statement.

> break;
> case MSR_SMI_COUNT:
> if (!msr_info->host_initiated)
> --

From: Sean Christopherson <seanjc@google.com>
Date: Fri, 4 Aug 2023 08:48:03 -0700
Subject: [PATCH] KVM: x86: Rework cpuid_get_supported_xcr0() to operate on
vCPU data

Rework and rename cpuid_get_supported_xcr0() to explicitly operate on vCPU
state, i.e. on a vCPU's CPUID state. Prior to commit 275a87244ec8 ("KVM:
x86: Don't adjust guest's CPUID.0x12.1 (allowed SGX enclave XFRM)"), KVM
incorrectly fudged guest CPUID at runtime, which in turn necessitated
massaging the incoming CPUID state for KVM_SET_CPUID{2} so as not to run
afoul of kvm_cpuid_check_equal().

Opportunistically move the helper below kvm_update_cpuid_runtime() to make
it harder to repeat the mistake of querying supported XCR0 for runtime
updates.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 7f4d13383cf2..5e42846c948a 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -234,21 +234,6 @@ void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
vcpu->arch.pv_cpuid.features = best->eax;
}

-/*
- * Calculate guest's supported XCR0 taking into account guest CPUID data and
- * KVM's supported XCR0 (comprised of host's XCR0 and KVM_SUPPORTED_XCR0).
- */
-static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
-{
- struct kvm_cpuid_entry2 *best;
-
- best = cpuid_entry2_find(entries, nent, 0xd, 0);
- if (!best)
- return 0;
-
- return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
-}
-
static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
int nent)
{
@@ -299,6 +284,21 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_update_cpuid_runtime);

+/*
+ * Calculate guest's supported XCR0 taking into account guest CPUID data and
+ * KVM's supported XCR0 (comprised of host's XCR0 and KVM_SUPPORTED_XCR0).
+ */
+static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = kvm_find_cpuid_entry_index(vcpu, 0xd, 0);
+ if (!best)
+ return 0;
+
+ return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
+}
+
static bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
{
struct kvm_cpuid_entry2 *entry;
@@ -323,8 +323,7 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
kvm_apic_set_version(vcpu);
}

- vcpu->arch.guest_supported_xcr0 =
- cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
+ vcpu->arch.guest_supported_xcr0 = vcpu_get_supported_xcr0(vcpu);

/*
* FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if

base-commit: f0147fcfab840fe9a3f03e9645d25c1326373fe6
--