linux-kernel - Re: [PATCH v2 1/2] x86/sev/vc: fix efi runtime instruction emulation

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Message-ID: <6b4b8924-c0a7-58ab-f282-93d019cd0b96@amd.com>
Date: Mon, 23 Jun 2025 08:57:20 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Gerd Hoffmann <kraxel@...hat.com>, linux-coco@...ts.linux.dev
Cc: kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
 "H. Peter Anvin" <hpa@...or.com>,
 "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/sev/vc: fix efi runtime instruction emulation

On 6/2/25 05:50, Gerd Hoffmann wrote:
> In case efi_mm is active go use the userspace instruction decoder which
> supports fetching instructions from active_mm.  This is needed to make
> instruction emulation work for EFI runtime code, so it can use cpuid
> and rdmsr.
> 
> EFI runtime code uses the cpuid instruction to gather information about
> the environment it is running in, such as SEV being enabled or not, and
> choose (if needed) the SEV code path for ioport access.
> 
> EFI runtime code uses the rdmsr instruction to get the location of the
> CAA page (see SVSM spec, section 4.2 - "Post Boot").
> 
> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> ---
>  arch/x86/coco/sev/vc-handle.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
> index 0989d98da130..01c78182da88 100644
> --- a/arch/x86/coco/sev/vc-handle.c
> +++ b/arch/x86/coco/sev/vc-handle.c
> @@ -17,6 +17,7 @@
>  #include <linux/mm.h>
>  #include <linux/io.h>
>  #include <linux/psp-sev.h>
> +#include <linux/efi.h>
>  #include <uapi/linux/sev-guest.h>
>  
>  #include <asm/init.h>
> @@ -180,7 +181,7 @@ static enum es_result __vc_decode_kern_insn(struct es_em_ctxt *ctxt)
>  
>  static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
>  {
> -	if (user_mode(ctxt->regs))
> +	if (user_mode(ctxt->regs) || current->active_mm == &efi_mm)

A comment before this would be good. Something like:

/*
 * User instruction decoding is also required for the EFI runtime. Even
 * though EFI runtime is running in kernel mode, it uses special EFI
 * virtual address mappings that require the use of efi_mm to properly
 * address and decode.
 */

Might be too verbose, but that's the general idea.

Thanks,
Tom

>  		return __vc_decode_user_insn(ctxt);
>  	else
>  		return __vc_decode_kern_insn(ctxt);