linux-kernel - Re: Possible 5.19 regression for systems with 52-bit physical address support

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Message-ID: <YuLQJ53QGDzHqFGc@google.com>
Date:   Thu, 28 Jul 2022 18:06:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: Possible 5.19 regression for systems with 52-bit physical
 address support

On Thu, Jul 28, 2022, Michael Roth wrote:
> On Thu, Jul 28, 2022 at 02:56:50PM +0000, Sean Christopherson wrote:
> > On Thu, Jul 28, 2022, Michael Roth wrote:
> > Speaking of which, what prevents hardware (firmware?) from configuring the C-bit
> > position to be bit 51 and thus preventing KVM from generating the reserved #NPF?
> 
> I'm not sure if there's a way to change this: the related PPR documents
> the CPUID 0x8000001F as read-only along with the expected value, but
> it's not documented as 'fixed' so maybe there is some way.
> 
> However in this case, just like with Milan the C-bit position actually
> already is 51, but since for guests we rely on the value from
> boot_cpu_data.x86_phys_bits, which is less than 51, any bits in-between
> can be used to generate the RSVD bit in the exit field.

Ya, I forgot to include the "and MAXPHYADDR >= 50" clause. 

> So more problematic would be if boot_cpu_data.x86_phys_bits could be set
> to 51+, in which case we would silently break SEV-ES/SNP in a similar
> manner. That should probably just print an error and disable SEV-ES,
> similar to what should be done if mmio_caching is disabled in KVM
> module.

This is the scenario I'm curious about.  It's mostly a future problem, so I guess
I'm just wondering if there's a plan for making things work if/when this collision
occurs.