Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb hv call out of sev code

[Tianyu Lan <ltykernel@gmail.com>]

On 10/20/2021 9:56 PM, Tom Lendacky wrote:
> On 10/20/21 8:39 AM, Borislav Petkov wrote:
>> On Wed, Oct 20, 2021 at 08:39:59PM +0800, Tianyu Lan wrote:
>>> Hyper-V runs paravisor in guest VMPL0 which emulates some functions
>>> (e.g, timer, tsc, serial console and so on) via handling VC exception.
>>> GHCB pages are allocated and set up by the paravisor and report to Linux
>>> guest via MSR register.Hyper-V SEV implementation is unenlightened guest
>>> case which doesn't Linux doesn't handle VC and paravisor in the VMPL0
>>> handle it.
>>
>> Aha, unenlightened.
>>
>> So why don't you export the original function by doing this (only
>> partial diff to show intent only):

This follows Joreg's previous comment and I implemented similar version 
in the V! patchset([PATCH 05/13] HV: Add Write/Read MSR registers via 
ghcb page https://lkml.org/lkml/2021/7/28/668).
"Instead, factor out a helper function which contains what Hyper-V needs 
and use that in sev_es_ghcb_hv_call() and Hyper-V code."

https://lkml.org/lkml/2021/8/2/375

>>
>> ---
>> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
>> index f1d513897baf..bfe82f58508f 100644
>> --- a/arch/x86/kernel/sev-shared.c
>> +++ b/arch/x86/kernel/sev-shared.c
>> @@ -125,7 +125,7 @@ static enum es_result verify_exception_info(struct 
>> ghcb *ghcb, struct es_em_ctxt
>>       return ES_VMM_ERROR;
>>   }
>> -static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
>> +static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool 
>> set_ghcb_msr,
>>                         struct es_em_ctxt *ctxt,
>>                         u64 exit_code, u64 exit_info_1,
>>                         u64 exit_info_2)
>> @@ -138,7 +138,14 @@ static enum es_result sev_es_ghcb_hv_call(struct 
>> ghcb *ghcb,
>>       ghcb_set_sw_exit_info_1(ghcb, exit_info_1);
>>       ghcb_set_sw_exit_info_2(ghcb, exit_info_2);
>> -    sev_es_wr_ghcb_msr(__pa(ghcb));
>> +    /*
>> +     * Hyper-V unenlightened guests use a paravisor for communicating 
>> and
>> +     * GHCB pages are being allocated by that paravisor which uses a
>> +     * different MSR and protocol.
> 
> Just to clarify the comment, the paravisor uses the same GHCB MSR and 
> GHCB protocol, it just can't use __pa() to get the address of the GHCB. 
> So I expect that the Hyper-V support sets the address properly before 
> calling this function.
> 
> Thanks,
> Tom
> 
>> +     */
>> +    if (set_ghcb_msr)
>> +        sev_es_wr_ghcb_msr(__pa(ghcb));
>> +
>>       VMGEXIT();
>>       return verify_exception_info(ghcb, ctxt);
>>
>>