Re: [PATCH][v3] KVM: VMX: Enable XSAVE/XRSTORE for guest

Sheng Yang
2010-05-21T07:30:10Z

On Thursday 20 May 2010 17:46:40 Avi Kivity wrote:
> On 05/20/2010 12:16 PM, Sheng Yang wrote:
> > From: Dexuan Cui<dexuan.cui@intel.com>
> >
> > Enable XSAVE/XRSTORE for guest.
> >
> > Change from V2:
> > Addressed comments from Avi.
> >
> > Change from V1:
> >
> > 1. Use FPU API.
> > 2. Fix CPUID issue.
> > 3. Save/restore all possible guest xstate fields when switching. Because
> > we don't know which fields guest has already touched.
> >
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h
> > b/arch/x86/include/asm/kvm_host.h index d08bb4a..3938bd1 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -302,6 +302,7 @@ struct kvm_vcpu_arch {
> >
> > } update_pte;
> >
> > struct fpu guest_fpu;
> >
> > + u64 xcr0;
> >
> > gva_t mmio_fault_cr2;
> > struct kvm_pio_request pio;
> >
> > diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> > index 9e6779f..346ea66 100644
> > --- a/arch/x86/include/asm/vmx.h
> > +++ b/arch/x86/include/asm/vmx.h
> > @@ -266,6 +266,7 @@ enum vmcs_field {
> >
> > #define EXIT_REASON_EPT_VIOLATION 48
> > #define EXIT_REASON_EPT_MISCONFIG 49
> > #define EXIT_REASON_WBINVD 54
> >
> > +#define EXIT_REASON_XSETBV 55
> >
> > /*
> >
> > * Interruption-information format
> >
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 99ae513..a63f206 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -36,6 +36,8 @@
> >
> > #include<asm/vmx.h>
> > #include<asm/virtext.h>
> > #include<asm/mce.h>
> >
> > +#include<asm/i387.h>
> > +#include<asm/xcr.h>
> >
> > #include "trace.h"
> >
> > @@ -247,6 +249,9 @@ static const u32 vmx_msr_index[] = {
> >
> > };
> > #define NR_VMX_MSR ARRAY_SIZE(vmx_msr_index)
> >
> > +#define MERGE_TO_U64(low, high) \
> > + (((low)& -1u) | ((u64)((high)& -1u)<< 32))
> > +
>
> static inline u64 kvm_read_edx_eax(vcpu) in cache_regs.h
>
> > +static int handle_xsetbv(struct kvm_vcpu *vcpu)
> > +{
> > + u64 new_bv = MERGE_TO_U64(kvm_register_read(vcpu, VCPU_REGS_RAX),
> > + kvm_register_read(vcpu, VCPU_REGS_RDX));
> > +
> > + if (kvm_register_read(vcpu, VCPU_REGS_RCX) != 0)
> > + goto err;
> > + if (vmx_get_cpl(vcpu) != 0)
> > + goto err;
> > + if (!(new_bv& XSTATE_FP))
> > + goto err;
> > + if ((new_bv& XSTATE_YMM)&& !(new_bv& XSTATE_SSE))
> > + goto err;
>
> What about a check against unknown bits?
>
> > + vcpu->arch.xcr0 = new_bv;
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> > + skip_emulated_instruction(vcpu);
> > + return 1;
> > +err:
> > + kvm_inject_gp(vcpu, 0);
> > + return 1;
> > +}
> > +
> >
> > static int handle_apic_access(struct kvm_vcpu *vcpu)
> > {
> >
> > return emulate_instruction(vcpu, 0, 0, 0) == EMULATE_DONE;
> >
> > +static u64 host_xcr0;
>
> __read_mostly.
>
> > +
> > +static void update_cpuid(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_cpuid_entry2 *best;
> > +
> > + best = kvm_find_cpuid_entry(vcpu, 1, 0);
> > + if (!best)
> > + return;
> > +
> > + /* Update OSXSAVE bit */
> > + if (cpu_has_xsave&& best->function == 0x1) {
> > + best->ecx&= ~(bit(X86_FEATURE_OSXSAVE));
> > + if (kvm_read_cr4(vcpu)& X86_CR4_OSXSAVE)
> > + best->ecx |= bit(X86_FEATURE_OSXSAVE);
> > + }
> > +}
>
> Note: need to update after userspace writes cpuid as well.

Not quite understand. Userspace set OSXSAVE should be trimmed IMO...
>
> > +
> >
> > int __kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > {
> >
> > unsigned long old_cr4 = kvm_read_cr4(vcpu);
> >
> > @@ -481,6 +513,9 @@ int __kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned
> > long cr4)
> >
> > if (cr4& CR4_RESERVED_BITS)
> >
> > return 1;
> >
> > + if (!guest_cpuid_has_xsave(vcpu)&& (cr4& X86_CR4_OSXSAVE))
> > + return 1;
> > +
> >
> > if (is_long_mode(vcpu)) {
> >
> > if (!(cr4& X86_CR4_PAE))
> >
> > return 1;
> >
> > @@ -497,6 +532,9 @@ int __kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned
> > long cr4)
> >
> > if ((cr4 ^ old_cr4)& pdptr_bits)
> >
> > kvm_mmu_reset_context(vcpu);
> >
> > + if ((cr4 ^ old_cr4)& X86_CR4_OSXSAVE)
> > + update_cpuid(vcpu);
> > +
>
> I think we need to reload the guest's xcr0 at this point.
> Alternatively, call vmx_load_host_state() to ensure the the next entry
> will reload it.

Current xcr0 would be loaded when next vmentry.

And if we use prepare_guest_switch(), how about SVM?

>
> > @@ -1931,7 +1964,7 @@ static void do_cpuid_ent(struct kvm_cpuid_entry2
> > *entry, u32 function,
> >
> > switch (function) {
> >
> > case 0:
> > - entry->eax = min(entry->eax, (u32)0xb);
> > + entry->eax = min(entry->eax, (u32)0xd);
>
> Do we need any special handling for leaf 0xc?

Don't think so. CPUID would return all 0 for it.
>
> > @@ -4567,6 +4616,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >
> > kvm_x86_ops->prepare_guest_switch(vcpu);
> > if (vcpu->fpu_active)
> >
> > kvm_load_guest_fpu(vcpu);
> >
> > + if (kvm_read_cr4(vcpu)& X86_CR4_OSXSAVE)
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
>
> Better done in vmx_save_host_state(), so we only do it on context
> switches or entries from userspace.
>
> kvm_read_cr4_bits() is faster - doesn't need a vmcs_readl().
>
> > atomic_set(&vcpu->guest_mode, 1);
> > smp_wmb();
> >
> > @@ -5118,6 +5169,10 @@ void fx_init(struct kvm_vcpu *vcpu)
> >
> > fpu_alloc(&vcpu->arch.guest_fpu);
> > fpu_finit(&vcpu->arch.guest_fpu);
> >
> > + /* Ensure guest xcr0 is valid for loading */
> > + if (cpu_has_xsave)
> > + vcpu->arch.xcr0 = XSTATE_FP;
> > +
>
> Can do it unconditionally, not that it matters much.
>
> > void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> > {
> >
> > if (vcpu->guest_fpu_loaded)
> >
> > @@ -5134,6 +5197,10 @@ void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> >
> > vcpu->guest_fpu_loaded = 1;
> > unlazy_fpu(current);
> >
> > + /* Restore all possible states in the guest */
> > + if (cpu_has_xsave&& guest_cpuid_has_xsave(vcpu))
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK,
> > + cpuid_get_possible_xcr0(vcpu));
>
> Best to calculate it out of the fast path, when guest cpuid is set.
> Need to check it at this time as well.

You mean guest_cpuid_has_xsave()? Not quite understand the point here...

>
> Also can avoid it if guest xcr0 == host xcr0.

I don't know the assumption that "host use all possible xcr0 bits" can apply. If
so, only use host_xcr0 should be fine.

Would update other points. Thanks.
--
regards
Yang, Sheng
>
> > fpu_restore_checking(&vcpu->arch.guest_fpu);
> > trace_kvm_fpu(1);
> >
> > }
> >
> > @@ -5144,7 +5211,14 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> >
> > return;
> >
> > vcpu->guest_fpu_loaded = 0;
> >
> > + /* Save all possible states in the guest */
> > + if (cpu_has_xsave&& guest_cpuid_has_xsave(vcpu))
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK,
> > + cpuid_get_possible_xcr0(vcpu));
>
> Ditto.
>
> > fpu_save_init(&vcpu->arch.guest_fpu);
> >
> > + if (cpu_has_xsave)
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK,
> > + host_xcr0);
> >
> > ++vcpu->stat.fpu_reload;
> > set_bit(KVM_REQ_DEACTIVATE_FPU,&vcpu->requests);
> > trace_kvm_fpu(0);