linux-kernel - Re: [PATCH Part1 RFC v2 02/20] x86/sev: Save the negotiated GHCB version

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Date:   Tue, 11 May 2021 11:23:05 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, jroedel@...e.de, thomas.lendacky@....com,
        pbonzini@...hat.com, mingo@...hat.com, dave.hansen@...el.com,
        rientjes@...gle.com, seanjc@...gle.com, peterz@...radead.org,
        hpa@...or.com, tony.luck@...el.com
Subject: Re: [PATCH Part1 RFC v2 02/20] x86/sev: Save the negotiated GHCB
 version

On Fri, Apr 30, 2021 at 07:15:58AM -0500, Brijesh Singh wrote:
> The SEV-ES guest calls the sev_es_negotiate_protocol() to negotiate the
> GHCB protocol version before establishing the GHCB. Cache the negotiated
> GHCB version so that it can be used later.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/include/asm/sev.h   |  2 +-
>  arch/x86/kernel/sev-shared.c | 15 ++++++++++++---
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index fa5cd05d3b5b..7ec91b1359df 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -12,7 +12,7 @@
>  #include <asm/insn.h>
>  #include <asm/sev-common.h>
>  
> -#define GHCB_PROTO_OUR		0x0001UL
> +#define GHCB_PROTOCOL_MIN	1ULL
>  #define GHCB_PROTOCOL_MAX	1ULL
>  #define GHCB_DEFAULT_USAGE	0ULL
>  
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 6ec8b3bfd76e..48a47540b85f 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -14,6 +14,13 @@
>  #define has_cpuflag(f)	boot_cpu_has(f)
>  #endif
>  
> +/*
> + * Since feature negotitation related variables are set early in the boot
> + * process they must reside in the .data section so as not to be zeroed
> + * out when the .bss section is later cleared.

  *
  * GHCB protocol version negotiated with the hypervisor.
  */

> +static u16 ghcb_version __section(".data") = 0;

Did you not see this when running checkpatch.pl on your patch?

ERROR: do not initialise statics to 0
#141: FILE: arch/x86/kernel/sev-shared.c:22:
+static u16 ghcb_version __section(".data") = 0;

>  static bool __init sev_es_check_cpu_features(void)
>  {
>  	if (!has_cpuflag(X86_FEATURE_RDRAND)) {
> @@ -54,10 +61,12 @@ static bool sev_es_negotiate_protocol(void)
>  	if (GHCB_MSR_INFO(val) != GHCB_MSR_SEV_INFO_RESP)
>  		return false;
>  
> -	if (GHCB_MSR_PROTO_MAX(val) < GHCB_PROTO_OUR ||
> -	    GHCB_MSR_PROTO_MIN(val) > GHCB_PROTO_OUR)
> +	if (GHCB_MSR_PROTO_MAX(val) < GHCB_PROTOCOL_MIN ||
> +	    GHCB_MSR_PROTO_MIN(val) > GHCB_PROTOCOL_MAX)
>  		return false;
>  
> +	ghcb_version = min_t(size_t, GHCB_MSR_PROTO_MAX(val), GHCB_PROTOCOL_MAX);

How is that even supposed to work? GHCB_PROTOCOL_MAX is 1 so
ghcb_version will be always 1 when you do min_t() on values one of which
is 1.

Maybe I'm missing something...

-- 
Regards/Gruss,
    Boris.

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