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

[Michael Roth]

On Thu, Jul 28, 2022 at 02:56:50PM +0000, Sean Christopherson wrote:
> On Thu, Jul 28, 2022, Michael Roth wrote:
> > On Thu, Jul 28, 2022 at 08:44:30AM -0500, Michael Roth wrote:
> > > Hi Sean,
> > > 
> > > With this patch applied, AMD processors that support 52-bit physical
> > 
> > Sorry, threading got messed up. This is in reference to:
> > 
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20220420002747.3287931-1-seanjc%40google.com%2F%23r&data=05%7C01%7Cmichael.roth%40amd.com%7Cb0cbbc83a88e4aca870008da70a96a80%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637946170190371699%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=KUBKPThOGMP36SpN3OCkJKeymkpkh5tK%2BJJv7ExUI6w%3D&reserved=0
> > 
> > commit 8b9e74bfbf8c7020498a9ea600bd4c0f1915134d
> > Author: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Date:   Wed Apr 20 00:27:47 2022 +0000
> > 
> >     KVM: x86/mmu: Use enable_mmio_caching to track if MMIO caching is enabled
> 
> Oh crud.  I suspect I also broke EPT with MAXPHYADDR=52; the initial
> kvm_mmu_reset_all_pte_masks() will clear the flag, and it won't get set back to
> true even though EPT can generate a reserved bit fault.
> 
> > > address will result in MMIO caching being disabled. This ends up
> > > breaking SEV-ES and SNP, since they rely on the MMIO reserved bit to
> > > generate the appropriate NAE MMIO exit event.
> > >
> > > This failure can also be reproduced on Milan by disabling mmio_caching
> > > via KVM module parameter.
> 
> Hrm, this is a separate bug of sorts.  SEV-ES (and later) needs to have an explicit
> check the MMIO caching is enabled, e.g. my bug aside, if KVM can't use MMIO caching
> due to the location of the C-bit, then SEV-ES must be disabled.

Ok, make sense.

> 
> 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.

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.

> 
> > > In the case of AMD, guests use a separate physical address range that
> > > and so there are still reserved bits available to make use of the MMIO
> > > caching. This adjustment happens in svm_adjust_mmio_mask(), but since
> > > mmio_caching_enabled flag is 0, any attempts to update masks get
> > > ignored by kvm_mmu_set_mmio_spte_mask().
> > > 
> > > Would adding 'force' parameter to kvm_mmu_set_mmio_spte_mask() that
> > > svm_adjust_mmio_mask() can set to ignore enable_mmio_caching be
> > > reasonable fix, or should we take a different approach?
> 
> Different approach.  To fix the bug with enable_mmio_caching not being set back to
> true when a vendor-specific mask allows caching, I believe the below will do the
> trick.
> 
> The SEV-ES dependency is easy to solve, but will require a few patches in order
> to get the necessary ordering; svm_adjust_mmio_mask() is currently called _after_
> SEV-ES is configured.
> 
> I'll test (as much as I can, I don't think we have platforms with MAXPHYADDR=52)
> and get a series sent out later today.

Will make sure to give this a test on my system as soon as it goes out.

Thanks!

-Mike