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

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Message-ID: <a1b2eba5-243c-4c7c-9ebd-3fce6cd4c973@amd.com>
Date: Wed, 9 Oct 2024 11:31:07 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
 dave.hansen@...ux.intel.com, Thomas.Lendacky@....com, nikunj@....com,
 Santosh.Shukla@....com, Vasant.Hegde@....com, Suravee.Suthikulpanit@....com,
 David.Kaplan@....com, x86@...nel.org, hpa@...or.com, peterz@...radead.org,
 seanjc@...gle.com, pbonzini@...hat.com, kvm@...r.kernel.org
Subject: Re: [RFC 01/14] x86/apic: Add new driver for Secure AVIC

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.

- Neeraj

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