Re: [PATCH v4 03/24] x86/sev: Use MSR protocol only for early SVSM PVALIDATE call

[Tom Lendacky <thomas.lendacky@amd.com>]

On 7/9/25 03:08, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The early page state change API performs an SVSM call to PVALIDATE each
> page when running under a SVSM, and this involves either a GHCB page
> based call or a call based on the MSR protocol.
> 
> The GHCB page based variant involves VA to PA translation of the GHCB
> address, and this is best avoided in the startup code, where virtual
> addresses are ambiguous (1:1 or kernel virtual).
> 
> As this is the last remaining occurrence of svsm_perform_call_protocol()
> in the startup code, switch to the MSR protocol exclusively in this
> particular case, so that the GHCB based plumbing can be moved out of the
> startup code entirely in a subsequent patch.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

With some comments below.

> ---
>  arch/x86/boot/compressed/sev.c     | 20 --------------------
>  arch/x86/boot/startup/sev-shared.c |  4 +++-
>  2 files changed, 3 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> index fd1b67dfea22..b71c1ab6a282 100644
> --- a/arch/x86/boot/compressed/sev.c
> +++ b/arch/x86/boot/compressed/sev.c
> @@ -50,31 +50,11 @@ u64 svsm_get_caa_pa(void)
>  	return boot_svsm_caa_pa;
>  }
>  
> -int svsm_perform_call_protocol(struct svsm_call *call);
> -
>  u8 snp_vmpl;
>  
>  /* Include code for early handlers */
>  #include "../../boot/startup/sev-shared.c"
>  
> -int svsm_perform_call_protocol(struct svsm_call *call)
> -{
> -	struct ghcb *ghcb;
> -	int ret;
> -
> -	if (boot_ghcb)
> -		ghcb = boot_ghcb;
> -	else
> -		ghcb = NULL;
> -
> -	do {
> -		ret = ghcb ? svsm_perform_ghcb_protocol(ghcb, call)
> -			   : svsm_perform_msr_protocol(call);
> -	} while (ret == -EAGAIN);
> -
> -	return ret;
> -}
> -
>  static bool sev_snp_enabled(void)
>  {
>  	return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
> diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
> index 992abfa50508..1bb4b522dfaa 100644
> --- a/arch/x86/boot/startup/sev-shared.c
> +++ b/arch/x86/boot/startup/sev-shared.c
> @@ -753,7 +753,9 @@ static void __head svsm_pval_4k_page(unsigned long paddr, bool validate)
>  	call.rax = SVSM_CORE_CALL(SVSM_CORE_PVALIDATE);
>  	call.rcx = pc_pa;
>  
> -	ret = svsm_perform_call_protocol(&call);
> +	do {
> +		ret = svsm_perform_msr_protocol(&call);
> +	} while (ret == -EAGAIN);

There's no comment here, like in the previous change, about why we are
using this method directly. Can one be added?

Also, this makes the second place where this do-while call to the MSR
protocol is used, maybe a common function would be good to have now.

Thanks,
Tom

>  	if (ret)
>  		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
>