Re: [PATCH v3 16/22] kvm: x86: Add XCR0 support for Intel AMX
From: Sean Christopherson <seanjc@google.com>
Date: 2021-12-29 00:21:14
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kvm, linux-kselftest, lkml
On Wed, Dec 22, 2021, Jing Liu wrote:
Two XCR0 bits are defined for AMX to support XSAVE mechanism. Bit 17 is for tilecfg and bit 18 is for tiledata. The value of XCR0[17:18] is always either 00b or 11b.
Is that an SDM requirement, or an arbitrary Linux/KVM requirement?
Also, SDM recommends that only 64-bit operating systems enable Intel AMX by setting XCR0[18:17]. If a 32-bit guest tries to set dynamic bits, it
This is wrong. It has nothing to do with 32-bit guests. The restriction is on 32-bit _host kernels_, which I'm guessing never set the tile bits in _host_ XCR0.
quoted hunk ↗ jump to hunk
fails to pass vcpu->arch.guest_supported_xcr0 check and gets a #GP. Signed-off-by: Yang Zhong <redacted> Signed-off-by: Jing Liu <redacted> --- arch/x86/kvm/x86.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a48a89f73027..c558c098979a 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c@@ -210,7 +210,7 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs; #define KVM_SUPPORTED_XCR0 (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \ | XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \ | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \ - | XFEATURE_MASK_PKRU) + | XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE) u64 __read_mostly host_efer; EXPORT_SYMBOL_GPL(host_efer);@@ -990,6 +990,12 @@ static int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr) if ((xcr0 & XFEATURE_MASK_AVX512) != XFEATURE_MASK_AVX512) return 1; } + +#ifdef CONFIG_X86_64
Drop the #ifdef, it adds no meaningful value and requires the reader to think far harder than they should have. Yes, it's technically dead code for 32-bit KVM, but no one cares about performance of 32-bit KVM, and in any case it's extremely unlikely this will be anything but noise.
+ if ((xcr0 & XFEATURE_MASK_XTILE) && + ((xcr0 & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE)) + return 1; +#endif vcpu->arch.xcr0 = xcr0; if ((xcr0 ^ old_xcr0) & XFEATURE_MASK_EXTEND) -- 2.27.0