Re: [PATCH] arm64: swiotlb: Keep the default size for protected guests

[Robin Murphy <robin.murphy@arm.com>]

On 10/08/2026 2:08 pm, Jason Gunthorpe wrote:
> On Mon, Aug 10, 2026 at 12:46:51PM +0100, Will Deacon wrote:
>> On Mon, Aug 10, 2026 at 01:37:11PM +0200, Marek Szyprowski wrote:
>>> On 10.08.2026 12:20, Will Deacon wrote:
>>>> On Mon, Aug 10, 2026 at 02:59:42PM +0530, Aneesh Kumar K.V wrote:
>>>>> Marek Szyprowski <m.szyprowski@samsung.com> writes:
>>>>>
>>>>>> On 07.08.2026 20:20, Jason Gunthorpe wrote:
>>>>>>> On Fri, Aug 07, 2026 at 06:13:49PM +0000, Mostafa Saleh wrote:
>>>>>>>> But the whole thing is best effort anyway, the kernel picks
>>>>>>>> IO_TLB_DEFAULT_SIZE which does not depend on the system topology or
>>>>>>>> how many devices or how much DMA they do.
>>>>>>>> SWIOTLB memory is wasted if unused so we should be careful around
>>>>>>>> that as it would be the other way around and users would have to
>>>>>>>> decrease it manually.
>>>>>>> Yeah, it is why the arch code shouldn't really be sizing it directly,
>>>>>>> it should be done in common code and, yes, we are probably going to
>>>>>>> have to do something alot smarter to have the common code better
>>>>>>> auto-tune this for the CC case..
>>>>>> What about the $subject patch? I assume that it is still needed to
>>>>>>
>>>>>> restore the behavior that was altered by the "[PATCH v8 00/23]
>>>>>>
>>>>>> dma-mapping: Track shared DMA state through direct, pool and swiotlb
>>>>>>
>>>>>> paths?" patchset?
>>>>>>
>>>>> I would request that we pick this patch to fix the regression described
>>>>> in https://lore.kernel.org/all/yq5azeyxyfol.fsf@kernel.org/.
>>>> I really don't think we need it. CCA hardware isn't exactly widespread
>>>> and the KVM host side patches don't appear close to being merged.
>>>
>>> Does this mean that the branch for-next/coco [1] won't go to v7.3-rc1?
> 
> Let's not make progress on guest support contingent on KVM CCA host
> side patches please. I expect the CSPs will have VM instance types
> available based on CCA within quarters, and Linux as a Guest should
> work in those environments regardless of what KVM is doing. I don't
> really expect full KVM support for years, frankly, the patchset is
> massive. Even Intel and AMD don't have full KVM support yet.
> 
> People already have CCA capable HW, are already testing this stuff and
> the closer upstream can get to being workable as a guest without a
> mountain of OOT patches the better.
> 
> I agree the thing is not ideal, but it was merged to ARM like this a
> long time ago, this patch is just fixing a small oopsie (was it a
> merge conflict?) to put it back. I don't the objection.

Yup, it seems pretty clearly like a straightforward bug in this series 
(or maybe even just the merge resolution), where it should have just 
removed the use of SWIOTLB_FORCE, but changing the if/else structure 
inadvertently upset the whole flow in a way that it shouldn't have.

For a fix patch it might be clearer to restore the "(is_realm_world() || 
is_protected_kvm_guest())" condition exactly as before, then save any 
further refactoring for the next round of new development. And if there 
is a concern that skipping the resizing wastes memory for pKVM, then 
surely that falls on e62decaf98e7 ("arm64/coco: Add pKVM as a CC 
platform") which intentionally added that logic.

I do concur that there's not necessarily a mad panic to get this into 
Marek's 7.3 pull, as folks trying to use linux-next or bleeding-edge 
mainline for CCA work (or indeed anything) should know the risks, but it 
should at least be one for the 7.3-rc fixes cycle. Aneesh, FYI generally 
once things are queued, please just send follow-up fixes as their own 
thing rather than replies, for maximum clarity.

Thanks,
Robin.