Re: [RFC PATCH kernel] iommufd: Allow mapping from KVM's guest_memfd

Alexey Kardashevskiy
2026-02-26T06:48:57Z

On 26/2/26 00:55, Sean Christopherson wrote:
> On Wed, Feb 25, 2026, Alexey Kardashevskiy wrote:
>> For the new guest_memfd type, no additional reference is taken as
>> pinning is guaranteed by the KVM guest_memfd library.
>>
>> There is no KVM-GMEMFD->IOMMUFD direct notification mechanism as
>> the assumption is that:
>> 1) page stage change events will be handled by VMM which is going
>> to call IOMMUFD to remap pages;
>> 2) shrinking GMEMFD equals to VM memory unplug and VMM is going to
>> handle it.
>
> The VMM is outside of the kernel's effective TCB. Assuming the VMM will always
> do the right thing is a non-starter.

Right.

But, say, for 1), VMM does not the right thing and skips on PSC - the AMD host will observe IOMMU fault events - noisy but harmless. I wonder if it is different for others though.

Truncating gmemfd is bad, is having gmemfd->iommufd notification going to be enough for a starter? Thanks,

--
Alexey