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Subject: Re: [PATCH v6 14/42] x86/sev: Register GHCB memory when SEV-SNP is
 active
To:     Borislav Petkov <bp@alien8.de>
References: <20211008180453.462291-1-brijesh.singh@amd.com>
 <20211008180453.462291-15-brijesh.singh@amd.com> <YYFs+5UUMfyDgh/a@zn.tnic>
 <aea0e0c8-7f03-b9db-3084-f487a233c50b@amd.com> <YYGGv6EtWrw7cnLA@zn.tnic>
From:   Brijesh Singh <brijesh.singh@amd.com>
Message-ID: <a975dfbf-f9bb-982e-9814-7259bc075b71@amd.com>
Date:   Wed, 3 Nov 2021 15:10:16 -0500
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Hi Boris,


On 11/2/21 1:44 PM, Borislav Petkov wrote:
> On Tue, Nov 02, 2021 at 01:24:01PM -0500, Brijesh Singh wrote:
>> To answer your question, GHCB is registered at the time of first #VC
>> handling by the second exception handler.
> 
> And this is what I don't like - register at use. Instead of init
> everything *before* use.
> 
>> Mike can correct me, the CPUID page check is going to happen on first
>> #VC handling inside the early exception handler (i.e case 1).
> 
> What is the "CPUID page check"?
> 
> And no, you don't want to do any detection when an exception happens -
> you want to detect *everything* *first* and then do exceptions.
> 
>> See if my above explanation make sense. Based on it, I don't think it
>> makes sense to register the GHCB during the CPUID page detection. The
>> CPUID page detection will occur in early VC handling.
> 
> See above. If this needs more discussion, we can talk on IRC.
> 

Looking at the secondary CPU bring up path it seems that we will not be 
getting #VC until the early_setup_idt() is called. I am thinking to add 
function to register the GHCB from the early_setup_idt()

early_setup_idt()
{
   ...
   if (IS_ENABLED(CONFIG_MEM_ENCRYPT))
     sev_snp_register_ghcb()
   ...
}

The above will cover the APs and for BSP case I can call the same 
function just after the final IDT is loaded

cpu_init_exception_handling()
{
    ...
    ...
    /* Finally load the IDT */
    load_current_idt();

    if (IS_ENABLED(CONFIG_MEM_ENCRYPT))
      sev_snp_register_ghcb()

}

Please let me know if something like above is acceptable.

thanks