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

[David Hildenbrand]

On 18.03.25 20:27, Catalin Marinas wrote:
> On Tue, Mar 18, 2025 at 09:55:27AM -0300, Jason Gunthorpe wrote:
> > 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 don't think we should allow this.
>
> > 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.
>
> We have the vma->vm_page_prot when the memslot is added, so we could use
> this instead of additional KVM flags.

I thought we try to avoid messing with the VMA when adding memslots; 
because KVM_CAP_SYNC_MMU allows user space for changing the VMAs 
afterwards without changing the memslot?

include/uapi/linux/kvm.h:#define KVM_CAP_SYNC_MMU 16  /* Changes to host 
mmap are reflected in guest */

--
Cheers,

David / dhildenb