Re: [PATCH 10/11] KVM: SVM: move MSR_IA32_SPEC_CTRL save/restore to assembly

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 09, 2022, Paolo Bonzini wrote:
> Restoration of the host IA32_SPEC_CTRL value is probably too late
> with respect to the return thunk training sequence.
> 
> With respect to the user/kernel boundary, AMD says, "If software chooses
> to toggle STIBP (e.g., set STIBP on kernel entry, and clear it on kernel
> exit), software should set STIBP to 1 before executing the return thunk
> training sequence." I assume the same requirements apply to the guest/host
> boundary. The return thunk training sequence is in vmenter.S, quite close
> to the VM-exit. On hosts without V_SPEC_CTRL, however, the host's
> IA32_SPEC_CTRL value is not restored until much later.
> 
> To avoid this, move the restoration of host SPEC_CTRL to assembly and,
> for consistency, move the restoration of the guest SPEC_CTRL as well.
> This is not particularly difficult, apart from some care to cover both
> 32- and 64-bit, and to share code between SEV-ES and normal vmentry.
> 
> Cc: stable@vger.kernel.org
> Fixes: a149180fbcf3 ("x86: Add magic AMD return-thunk")
> Suggested-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

> +.ifnc _ASM_ARG1, _ASM_DI
> +	/*
> +	 * Stash @svm in RDI early. On 32-bit, arguments are in RAX, RCX
> +	 * and RDX which are clobbered by RESTORE_GUEST_SPEC_CTRL.
> +	 */
> +	mov %_ASM_ARG1, %_ASM_DI
> +.endif

Not technically needed since SEV-ES is 64-bit only, but that's a pre-exisiting
mess.  I'll send a follow-up patch to #ifdef out the entire function and drop all
of this internal ifdeffery, and provide a stub in C code stub in C code so that
32-bit can link (and kill the VM if if the stub is reached).