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 13:29:00 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, 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 5/11/21 4:23 AM, Borislav Petkov wrote:
> 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;
>
I ignored it, because I thought I still need to initialize the value to
zero because it does not go into .bss section which gets initialized to
zero by kernel on boot.

I guess I was wrong, maybe compiler or kernel build ensures that
ghcb_version variable to be init'ed to zero because its marked static ?


>>  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...

In patch #4, you will see that I increase the GHCB max protocol version
to 2, and then min_t() will choose the lower value. I didn't combine
version bump and caching into same patch because I felt I will be asked
to break them into two logical patches.


>