Re: [PATCH 1/4] x86: Reject CPU policies with vendors other than the host's

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/01/2026 4:49 pm, Alejandro Vallejo wrote:
> diff --git a/CHANGELOG.md b/CHANGELOG.md
> index 18f3d10f20..eae2f961c7 100644
> --- a/CHANGELOG.md
> +++ b/CHANGELOG.md
> @@ -22,6 +22,10 @@ The format is based on [Keep a 
> Changelog](https://keepachangelog.com/en/1.0.0/)
>     - Xenoprofile support.  Oprofile themselves removed support for Xen in 
> 2014
>       prior to the version 1.0 release, and there has been no development 
> since
>       before then in Xen.
> +   - Cross-vendor support.  Refuse to start domains whose CPU vendor differs
> +     from the host so that security mitigations stay consistent. Cross-vendor
> +     setups have been unreliable and not practical since 2017 with the 
> advent of
> +     speculation security.

This is going to want expanding upon, but there's a subtle change in
patch 4 needing addressing first.

> diff --git a/xen/lib/x86/policy.c b/xen/lib/x86/policy.c
> index f033d22785..4c0c5386ea 100644
> --- a/xen/lib/x86/policy.c
> +++ b/xen/lib/x86/policy.c
> @@ -15,7 +15,8 @@ int x86_cpu_policies_are_compatible(const struct cpu_policy 
> *host,
>  #define FAIL_MSR(m) \
>      do { e.msr = (m); goto out; } while ( 0 )
>  
> -    if ( guest->basic.max_leaf > host->basic.max_leaf )
> +    if ( (guest->basic.max_leaf >  host->basic.max_leaf) ||
> +         (guest->x86_vendor     != host->x86_vendor) )
>          FAIL_CPUID(0, NA);

    if ( guest->x86_vendor     != host->x86_vendor ||
         guest->basic.max_leaf >  host->basic.max_leaf )

please.  This function is going to get much much longer when we're done
with it, and I'd like to try and keep the checks in the right cognitive
order.

~Andrew