Re: [PATCH 2/4] x86/hvm: Disable non-FEP cross-vendor handling in #UD handler

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Thu Jan 22, 2026 at 5:49 PM CET, Alejandro Vallejo wrote:
> Remove cross-vendor support now that VMs can no longer have a different
> vendor than the host, leaving FEP as the sole raison-d'être for #UD
> interception.
>
> Not a functional change.
>
> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
> ---
>  xen/arch/x86/hvm/hvm.c     | 25 ++++---------------------
>  xen/arch/x86/hvm/svm/svm.c |  4 ++--
>  xen/arch/x86/hvm/vmx/vmx.c |  4 ++--
>  3 files changed, 8 insertions(+), 25 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 4d37a93c57..611ff83a60 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -3832,28 +3832,13 @@ int hvm_descriptor_access_intercept(uint64_t 
> exit_info,
>      return X86EMUL_OKAY;
>  }
>  
> -static bool cf_check is_cross_vendor(
> -    const struct x86_emulate_state *state, const struct x86_emulate_ctxt 
> *ctxt)
> -{
> -    switch ( ctxt->opcode )
> -    {
> -    case X86EMUL_OPC(0x0f, 0x05): /* syscall */
> -    case X86EMUL_OPC(0x0f, 0x34): /* sysenter */
> -    case X86EMUL_OPC(0x0f, 0x35): /* sysexit */
> -        return true;
> -    }
> -
> -    return false;
> -}
> -
> +#ifdef CONFIG_HVM_FEP
>  void hvm_ud_intercept(struct cpu_user_regs *regs)
>  {
>      struct vcpu *cur = current;
> -    bool should_emulate =
> -        cur->domain->arch.cpuid->x86_vendor != boot_cpu_data.x86_vendor;
>      struct hvm_emulate_ctxt ctxt;
>  
> -    hvm_emulate_init_once(&ctxt, opt_hvm_fep ? NULL : is_cross_vendor, regs);
> +    hvm_emulate_init_once(&ctxt, NULL, regs);
>  
>      if ( opt_hvm_fep )
>      {
> @@ -3878,12 +3863,9 @@ void hvm_ud_intercept(struct cpu_user_regs *regs)
>                  regs->rip = (uint32_t)regs->rip;
>  
>              add_taint(TAINT_HVM_FEP);
> -
> -            should_emulate = true;
>          }
>      }
> -
> -    if ( !should_emulate )
> +    else

review to self. This is buggy. It allows instruction emulation when HVM_FEP is
enabled, but the FEP is absent in the particular instruction that caused the
exception.

#UD should be re-injected when the instruction doesn't have the prefix.

Cheers,
Alejandro