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

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

> 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@redhat.com>

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.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);