| Date | Tue, 12 Oct 2021 19:36:56 +0200 | Subject | Re: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user | From | Paolo Bonzini <> |
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On 12/10/21 02:00, Thomas Gleixner wrote: > > - if (boot_cpu_has(X86_FEATURE_XSAVE)) { > - memset(guest_xsave, 0, sizeof(struct kvm_xsave)); > - fill_xsave((u8 *) guest_xsave->region, vcpu); > - } else { > - memcpy(guest_xsave->region, > - &vcpu->arch.guest_fpu->state.fxsave, > - sizeof(struct fxregs_state)); > - *(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] = > - XFEATURE_MASK_FPSSE; > - }
After the patch, this final assignment is not done in the else case:
> + > + if (cpu_feature_enabled(X86_FEATURE_XSAVE)) { > + __copy_xstate_to_uabi_buf(mb, &kstate->xsave, pkru, > + XSTATE_COPY_XSAVE); > + } else { > + memcpy(&ustate->fxsave, &kstate->fxsave, sizeof(ustate->fxsave)); > + } > +}
This leaves the xstate_bv set to 0 instead of XFEATURE_MASK_FPSSE. Resuming a VM then fails if you save on a non-XSAVE machine and restore it on an XSAVE machine.
The memset(guest_xsave, 0, sizeof(struct kvm_xsave)) also is not reproduced, you can make it unconditional for simplicity; this is not a fast path.
Paolo
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