Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

[Jason Gunthorpe]

On Thu, May 15, 2025 at 05:57:57PM -0700, Sean Christopherson wrote:

> You're conflating two different things.  guest_memfd allocating and managing
> 1GiB physical pages, and KVM mapping memory into the guest at 1GiB/2MiB
> granularity.  Allocating memory in 1GiB chunks is useful even if KVM can only
> map memory into the guest using 4KiB pages.

Even if KVM is limited to 4K the IOMMU might not be - alot of these
workloads have a heavy IO component and we need the iommu to perform
well too.

Frankly, I don't think there should be objection to making memory more
contiguous. There is alot of data that this always brings wins
somewhere for someone.

> The longer term goal of guest_memfd is to make it suitable for backing all VMs,
> hence Vishal's "Non-CoCo VMs" comment.  Yes, some of this is useful for TDX, but
> we (and others) want to use guest_memfd for far more than just CoCo VMs.  And
> for non-CoCo VMs, 1GiB hugepages are mandatory for various workloads.

Yes, even from an iommu perspective with 2D translation we need to
have the 1G pages from the S2 resident in the IOTLB or performance
falls off a cliff.

Jason