Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

[Marc Zyngier]

On Mon, 17 Mar 2025 05:55:55 +0000,
Ankit Agrawal <ankita@xxxxxxxxxx> wrote:
> 
> >> For my education, what is an accepted way to communicate this? Please let
> >> me know if there are any relevant examples that you may be aware of.
> >
> > A KVM capability is what is usually needed.
> 
> I see. If IIUC, this would involve a corresponding Qemu (usermode) change
> to fetch the new KVM cap. Then it could fail in case the FWB is not
> supported with some additional conditions (so that the currently supported
> configs with !FWB won't break on usermode). 
> 
> The proposed code change is to map in S2 as NORMAL when vma flags
> has VM_PFNMAP. However, Qemu cannot know that driver is mapping
> with PFNMAP or not. So how may Qemu decide whether it is okay to
> fail for !FWB or not?

This is not about FWB as far as userspace is concerned. This is about
PFNMAP as non-device memory. If the host doesn't have FWB, then the
"PFNMAP as non-device memory" capability doesn't exist, and userspace
fails early.

Userspace must also have some knowledge of what device it obtains the
mapping from, and whether that device requires some extra host
capability to be assigned to the guest.

You can then check whether the VMA associated with the memslot is
PFNMAP or not, if the memslot has been enabled for PFNMAP mappings
(either globally or on a per-memslot basis, I don't really care).

	M.

-- 
Without deviation from the norm, progress is not possible.