Re: [RFC 01/14] x86/apic: Add new driver for Secure AVIC

[Tom Lendacky]

On 10/9/24 01:01, Neeraj Upadhyay wrote:
> 
> 
> On 10/9/2024 10:53 AM, Borislav Petkov wrote:
>> On Wed, Oct 09, 2024 at 07:26:55AM +0530, Neeraj Upadhyay wrote:
>>> As SECURE_AVIC feature is not supported (as reported by snp_get_unsupported_features())
>>> by guest at this patch in the series, it is added to SNP_FEATURES_IMPL_REQ here. The bit
>>> value within SNP_FEATURES_IMPL_REQ hasn't changed with this change as the same bit pos
>>> was part of MSR_AMD64_SNP_RESERVED_MASK before this patch. In patch 14 SECURE_AVIC guest
>>> support is indicated by guest.
>>
>> So what's the point of adding it to SNP_FEATURES_IMPL_REQ here? What does that
>> do at all in this patch alone? Why is this change needed in here?
>>
> 
> Before this patch, if hypervisor enables Secure AVIC  (reported in sev_status), guest would
> terminate in snp_check_features(). The reason for this is, SNP_FEATURES_IMPL_REQ had the Secure
> AVIC bit set before this patch, as that bit was part of MSR_AMD64_SNP_RESERVED_MASK 
> GENMASK_ULL(63, 18).
> 
> #define SNP_FEATURES_IMPL_REQ	(MSR_AMD64_SNP_VTOM |			\
> 				 ...
> 				 MSR_AMD64_SNP_RESERVED_MASK)
> 
> 
> 
> Adding MSR_AMD64_SNP_SECURE_AVIC_BIT (bit 18) to SNP_FEATURES_IMPL_REQ in this patch
> keeps that behavior intact as now with this change MSR_AMD64_SNP_RESERVED_MASK becomes
> GENMASK_ULL(63, 19).
> 
> 
>> IOW, why don't you do all the feature bit handling in the last patch, where it
>> all belongs logically?
>>
> 
> If we do that, then hypervisor could have enabled Secure AVIC support and the guest
> code at this patch won't catch the missing guest-support early and it can result in some
> unknown failures at later point during guest boot.

Won't the SNP_RESERVED_MASK catch it? You are just renaming the bit
position value, right? It was a 1 before and is still a 1. So the guest
will terminate if the hypervisor sets the Secure AVIC bit both before
and after this patch, right?

Thanks,
Tom

> 
> 
> - Neeraj
> 
>> In the last patch you can start *testing* for
>> MSR_AMD64_SNP_SECURE_AVIC_ENABLED *and* enforce it with SNP_FEATURES_PRESENT.
>>