Re: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user

Paolo Bonzini
2021-10-12T17:37:05Z

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