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

[Jason Gunthorpe]

On Tue, Mar 18, 2025 at 09:39:30AM +0000, Marc Zyngier wrote:

> The memslot must also be created with a new flag ((2c) in the taxonomy
> above) that carries the "Please map VM_PFNMAP VMAs as cacheable". This
> flag is only allowed if (1) is valid.
> 
> This results in the following behaviours:
> 
> - If the VMM creates the memslot with the cacheable attribute without
>   (1) being advertised, we fail.
> 
> - If the VMM creates the memslot without the cacheable attribute, we
>   map as NC, as it is today.

Is that OK though?

Now we have the MM page tables mapping this memory as cachable but KVM
and the guest is accessing it as non-cached.

I thought ARM tried hard to avoid creating such mismatches? This is
why the pgprot flags were used to drive this, not an opt-in flag. To
prevent userspace from forcing a mismatch.

> What this doesn't do is *automatically* decide for the VMM what
> attributes to use. The VMM must know what it is doing, and only
> provide the memslot flag when appropriate. Doing otherwise may eat
> your data and/or take the machine down (cacheable mapping on a device
> can be great fun).

Again, this is why we followed the VMA flags. The thing creating the
VMA already made this safety determination when it set pgprot
cachable. We should not allow KVM to randomly make any PGPROT
cachable!

> If you want to address this, then "someone" needs
> to pass some additional VMA flag that KVM can check.

pgprot does this already, we don't need a new flag to determine if the
VMA is cachably mapped.

Jason