[PATCH MANUALSEL 5.16 8/8] KVM: x86: Report deprecated x87 features in supported CPUID

Sasha Levin
2022-02-09T18:57:26Z

From: Jim Mattson <jmattson@google.com>

[ Upstream commit e3bcfda012edd3564e12551b212afbd2521a1f68 ]

CPUID.(EAX=7,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6] and
CPUID.(EAX=7,ECX=0):EBX.ZERO_FCS_FDS[bit 13] are "defeature"
bits. Unlike most of the other CPUID feature bits, these bits are
clear if the features are present and set if the features are not
present. These bits should be reported in KVM_GET_SUPPORTED_CPUID,
because if these bits are set on hardware, they cannot be cleared in
the guest CPUID. Doing so would claim guest support for a feature that
the hardware doesn't support and that can't be efficiently emulated.

Of course, any software (e.g WIN87EM.DLL) expecting these features to
be present likely predates these CPUID feature bits and therefore
doesn't know to check for them anyway.

Aaron Lewis added the corresponding X86_FEATURE macros in
commit cbb99c0f5887 ("x86/cpufeatures: Add FDP_EXCPTN_ONLY and
ZERO_FCS_FDS"), with the intention of reporting these bits in
KVM_GET_SUPPORTED_CPUID, but I was unable to find a proposed patch on
the kvm list.

Opportunistically reordered the CPUID_7_0_EBX capability bits from
least to most significant.

Cc: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20220204001348.2844660-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kvm/cpuid.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index add8f58d686e3..bf18679757c70 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -532,12 +532,13 @@ void kvm_set_cpu_caps(void)
);

kvm_cpu_cap_mask(CPUID_7_0_EBX,
- F(FSGSBASE) | F(SGX) | F(BMI1) | F(HLE) | F(AVX2) | F(SMEP) |
- F(BMI2) | F(ERMS) | F(INVPCID) | F(RTM) | 0 /*MPX*/ | F(RDSEED) |
- F(ADX) | F(SMAP) | F(AVX512IFMA) | F(AVX512F) | F(AVX512PF) |
- F(AVX512ER) | F(AVX512CD) | F(CLFLUSHOPT) | F(CLWB) | F(AVX512DQ) |
- F(SHA_NI) | F(AVX512BW) | F(AVX512VL) | 0 /*INTEL_PT*/
- );
+ F(FSGSBASE) | F(SGX) | F(BMI1) | F(HLE) | F(AVX2) |
+ F(FDP_EXCPTN_ONLY) | F(SMEP) | F(BMI2) | F(ERMS) | F(INVPCID) |
+ F(RTM) | F(ZERO_FCS_FDS) | 0 /*MPX*/ | F(AVX512F) |
+ F(AVX512DQ) | F(RDSEED) | F(ADX) | F(SMAP) | F(AVX512IFMA) |
+ F(CLFLUSHOPT) | F(CLWB) | 0 /*INTEL_PT*/ | F(AVX512PF) |
+ F(AVX512ER) | F(AVX512CD) | F(SHA_NI) | F(AVX512BW) |
+ F(AVX512VL));

kvm_cpu_cap_mask(CPUID_7_ECX,
F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
--
2.34.1