linux-kernel - Re: [PATCH v4 1/3] x86/sev/vc: fix efi runtime instruction emulation

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Message-ID: <d3189b65-ec6a-4dba-adac-44e72935419e@amd.com>
Date: Fri, 27 Jun 2025 10:33:28 +0200
From: "Gupta, Pankaj" <pankaj.gupta@....com>
To: Gerd Hoffmann <kraxel@...hat.com>, linux-coco@...ts.linux.dev,
 kvm@...r.kernel.org
Cc: 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 v4 1/3] x86/sev/vc: fix efi runtime instruction emulation


> 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").
> 
> The big picture behind this is that the kernel needs to be able to
> properly handle #VC exceptions that come from EFI runtime services.
> Since EFI runtime services have a special page table mapping for the EFI
> virtual address space, the efi_mm context must be used when decoding
> instructions during #VC handling.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>

Reviewed-by: Pankaj Gupta <pankaj.gupta@....com>

> ---
>   arch/x86/coco/sev/vc-handle.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
> index 0989d98da130..faf1fce89ed4 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>
> @@ -178,9 +179,15 @@ static enum es_result __vc_decode_kern_insn(struct es_em_ctxt *ctxt)
>   		return ES_OK;
>   }
>   
> +/*
> + * User instruction decoding is also required for the EFI runtime. Even though
> + * the 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.
> + */
>   static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
>   {
> -	if (user_mode(ctxt->regs))
> +	if (user_mode(ctxt->regs) || mm_is_efi(current->active_mm))
>   		return __vc_decode_user_insn(ctxt);
>   	else
>   		return __vc_decode_kern_insn(ctxt);