On Tue, 2022-08-16 at 21:37 +0000, Sean Christopherson wrote: > On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote: > > From: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> > > > > A live migration under qemu is currently failing when the source > > host is ~Nehalem era (pre-xsave) and the destination is much newer, > > (configured with a guest CPU type of Nehalem). > > QEMU always calls kvm_put_xsave, even on this combination because > > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE. > > > > When QEMU calls kvm_put_xsave it's rejected by > > fpu_copy_uabi_to_guest_fpstate-> > > copy_uabi_to_xstate-> > > validate_user_xstate_header > > > > when the validate checks the loaded xfeatures against > > user_xfeatures, which it finds to be 0. > > > > I think our initialisation of user_xfeatures is being > > too strict here, and we should always allow the base FP/SSE. > > > > Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0") Thanks for fixing this, Dave! > > bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311 > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> > > --- > > arch/x86/kvm/cpuid.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > index de6d44e07e34..3b2319cecfd1 100644 > > --- a/arch/x86/kvm/cpuid.c > > +++ b/arch/x86/kvm/cpuid.c > > @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) > > guest_supported_xcr0 = > > cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent); > > > > - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0; > > + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 | > > + XFEATURE_MASK_FPSSE; > > I don't think this is correct. This will allow the guest to set the SSE bit > even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning > user_xfeatures. > > static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu) > { > return vcpu->arch.guest_fpu.fpstate->user_xfeatures; > } > > I believe the right place to fix this is in validate_user_xstate_header(). It's > reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_ > supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel > should also allow FP+SSE when using XSAVE too. That would also align the logic > with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for > the non-KVM save_xstate_epilog(). > > Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE > is enabled (knew their had to be a sanity check somewhere). Thanks for the feedback Sean! I have near to no experience in this code, and I hope you can help me with a question I have, based in Dave's commit message: > > QEMU always calls kvm_put_xsave, even on this combination because > > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE. Any particular reason why it always returns true for KVM_CAP_XSAVE, even when the CPU does not support it? IIUC, if it returns false to this capability, kvm_put_xsave() should never be called, and thus it can avoid bug reproduction. Thanks in advance, Leo > > --- > arch/x86/kernel/fpu/xstate.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c > index c8340156bfd2..83b9a9653d47 100644 > --- a/arch/x86/kernel/fpu/xstate.c > +++ b/arch/x86/kernel/fpu/xstate.c > @@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr) > static int validate_user_xstate_header(const struct xstate_header *hdr, > struct fpstate *fpstate) > { > - /* No unknown or supervisor features may be set */ > - if (hdr->xfeatures & ~fpstate->user_xfeatures) > + /* > + * No unknown or supervisor features may be set. Userspace is always > + * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel > + * if and only if FP+SSE are supported in xstate). > + */ > + if (hdr->xfeatures & ~fpstate->user_xfeatures & > + ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE)) > return -EINVAL; > > /* Userspace must use the uncompacted format */ > > base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb > -- >