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. > fails to pass vcpu->arch.guest_supported_xcr0 check and gets a #GP. > > Signed-off-by: Yang Zhong <yang.zhong@xxxxxxxxx> > Signed-off-by: Jing Liu <jing2.liu@xxxxxxxxx> > --- > 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 >
