Re: [RFC Part1 PATCH 07/13] x86/compressed: register GHCB memory when SNP is active

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

On 4/7/21 12:34 PM, Brijesh Singh wrote:
> 
> On 4/7/21 6:59 AM, Borislav Petkov wrote:
>> On Wed, Mar 24, 2021 at 11:44:18AM -0500, Brijesh Singh wrote:
>>> The SEV-SNP guest is required to perform GHCB GPA registration. This is
>> Why does it need to do that? Some additional security so as to not allow
>> changing the GHCB once it is established?
>>
>> I'm guessing that's enforced by the SNP fw and we cannot do that
>> retroactively for SEV...? Because it sounds like a nice little thing we
>> could do additionally.
> 
> The feature is part of the GHCB version 2 and is enforced by the
> hypervisor. I guess it can be extended for the ES. Since this feature
> was not available in GHCB version 1 (base ES) so it should be presented
> as an optional for the ES ?

GHCB GPA registration is only supported and required for SEV-SNP guests.
The final version of the spec documents that and should be published
within the next few days.

Thanks,
Tom

> 
> 
>>
>>> because the hypervisor may prefer that a guest use a consistent and/or
>>> specific GPA for the GHCB associated with a vCPU. For more information,
>>> see the GHCB specification section 2.5.2.
>> I think you mean
>>
>> "2.3.2 GHCB GPA Registration"
>>
>> Please use the section name too because that doc changes from time to
>> time.
>>
>> Also, you probably should update it here:
>>
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D206537&data=04%7C01%7Cbrijesh.singh%40amd.com%7Ce8ae7574ecc742be6c1a08d8f9bcac94%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637533936070042328%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=NaHJ5R9Dfo%2FPnci%2B%2B6xK9ecpV0%2F%2FYbsdGl25%2BFj3TaU%3D&reserved=0
>>
> 
> Yes, the section may have changed since I wrote the description. Noted.
> I will refer the section name.
> 
> 
>>> diff --git a/arch/x86/boot/compressed/sev-snp.c b/arch/x86/boot/compressed/sev-snp.c
>>> index 5c25103b0df1..a4c5e85699a7 100644
>>> --- a/arch/x86/boot/compressed/sev-snp.c
>>> +++ b/arch/x86/boot/compressed/sev-snp.c
>>> @@ -113,3 +113,29 @@ void sev_snp_set_page_shared(unsigned long paddr)
>>>  {
>>>  	sev_snp_set_page_private_shared(paddr, SNP_PAGE_STATE_SHARED);
>>>  }
>>> +
>>> +void sev_snp_register_ghcb(unsigned long paddr)
>> Right and let's prefix SNP-specific functions with "snp_" only so that
>> it is clear which is wcich when looking at the code.
>>
>>> +{
>>> +	u64 pfn = paddr >> PAGE_SHIFT;
>>> +	u64 old, val;
>>> +
>>> +	if (!sev_snp_enabled())
>>> +		return;
>>> +
>>> +	/* save the old GHCB MSR */
>>> +	old = sev_es_rd_ghcb_msr();
>>> +
>>> +	/* Issue VMGEXIT */
>> No need for that comment.
>>
>>> +	sev_es_wr_ghcb_msr(GHCB_REGISTER_GPA_REQ_VAL(pfn));
>>> +	VMGEXIT();
>>> +
>>> +	val = sev_es_rd_ghcb_msr();
>>> +
>>> +	/* If the response GPA is not ours then abort the guest */
>>> +	if ((GHCB_SEV_GHCB_RESP_CODE(val) != GHCB_REGISTER_GPA_RESP) ||
>>> +	    (GHCB_REGISTER_GPA_RESP_VAL(val) != pfn))
>>> +		sev_es_terminate(GHCB_SEV_ES_REASON_GENERAL_REQUEST);
>> Yet another example where using a specific termination reason could help
>> with debugging guests. Looking at the GHCB spec, I hope GHCBData[23:16]
>> is big enough for all reasons. I'm sure it can be extended ofc ...
> 
> 
> Maybe we can request the GHCB version 3 to add the extended error code.
> 
> 
>> :-)
>>
>>> +	/* Restore the GHCB MSR value */
>>> +	sev_es_wr_ghcb_msr(old);
>>> +}
>>> diff --git a/arch/x86/include/asm/sev-snp.h b/arch/x86/include/asm/sev-snp.h
>>> index f514dad276f2..0523eb21abd7 100644
>>> --- a/arch/x86/include/asm/sev-snp.h
>>> +++ b/arch/x86/include/asm/sev-snp.h
>>> @@ -56,6 +56,13 @@ struct __packed snp_page_state_change {
>>>  	struct snp_page_state_entry entry[SNP_PAGE_STATE_CHANGE_MAX_ENTRY];
>>>  };
>>>  
>>> +/* GHCB GPA register */
>>> +#define GHCB_REGISTER_GPA_REQ	0x012UL
>>> +#define		GHCB_REGISTER_GPA_REQ_VAL(v)		(GHCB_REGISTER_GPA_REQ | ((v) << 12))
>>> +
>>> +#define GHCB_REGISTER_GPA_RESP	0x013UL
>> Let's append "UL" to the other request numbers for consistency.
>>
>> Thx.
>>