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

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Message-ID: <ed25f5a6-dad6-4b5e-b42b-58e6ced6c7a2@redhat.com>
Date: Tue, 18 Mar 2025 20:35:38 +0100
From: David Hildenbrand <david@...hat.com>
To: Catalin Marinas <catalin.marinas@....com>,
 Jason Gunthorpe <jgg@...dia.com>
Cc: Marc Zyngier <maz@...nel.org>, Ankit Agrawal <ankita@...dia.com>,
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Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

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