Message-ID: <20250710175858.GB1870174@ziepe.ca>
Date: Thu, 10 Jul 2025 14:58:58 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alexey Kardashevskiy <aik@....com>
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Subject: Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce
 KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls

On Thu, Jul 10, 2025 at 04:57:25PM +1000, Alexey Kardashevskiy wrote:

> Currently I handle this from the KVM with a hack to get IOPDE from
> AMD IOMMU so both 2MB RMP entry and IOPDE entries are smashed in one
> go in one of many firmwares running on EPYC, and atm this is too
> hacky to be posted even as an RFC. This likely needs to move to
> IOMMUFD then (via some callbacks) which could call AMD IOMMU which
> then would call that firmware (called "TMPM" and it is not the PSP
> which is "TSM), probably. Thanks,

Wasn't the issue with the iommu that it needed to have a PTE break
whenever the shared/private changed in the RMP? Because the HW can't
handle an IOPTE that crosses more than one RMP entry? Or do I
misunderstand the problem?

If this is the problem I was expecting the page table code that
translates the guest memfd into the iommu PTEs would respect the
shared/private conversion boundaries and break up the PTEs
automatically.

I had thought there were three versions of of how to copy from guest
memfd into the IOPTEs:
 - HW must never have a private physaddr in an IOPTE
 - HW must have IOPTEs entirely private or shared
 - HW handles everything and IOPTEs should be maximally sized

Is this right? Is AMD #2?

Jason

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