Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off

[Sean Christopherson <seanjc@google.com>]

On Fri, Jan 29, 2021, Paolo Bonzini wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 76bce832cade..15733013b266 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1401,7 +1401,7 @@ static u64 kvm_get_arch_capabilities(void)
>  	 *	  This lets the guest use VERW to clear CPU buffers.


This comment be updated to call out the new TSX_CTRL behavior.

	/*
	 * On TAA affected systems:
	 *      - nothing to do if TSX is disabled on the host.
	 *      - we emulate TSX_CTRL if present on the host.
	 *	  This lets the guest use VERW to clear CPU buffers.
	 */

>  	 */
>  	if (!boot_cpu_has(X86_FEATURE_RTM))
> -		data &= ~(ARCH_CAP_TAA_NO | ARCH_CAP_TSX_CTRL_MSR);
> +		data &= ~ARCH_CAP_TAA_NO;

Hmm, simply clearing TSX_CTRL will only preserve the host value.  Since
ARCH_CAPABILITIES is unconditionally emulated by KVM, wouldn't it make sense to
unconditionally expose TSX_CTRL as well, as opposed to exposing it only if it's
supported in the host?  I.e. allow migrating a TSX-disabled guest to a host
without TSX.  Or am I misunderstanding how TSX_CTRL is checked/used?

>  	else if (!boot_cpu_has_bug(X86_BUG_TAA))
>  		data |= ARCH_CAP_TAA_NO;
>  
> -- 
> 2.26.2
>