On Mon, Aug 10, 2026 at 03:14:47PM +0100, Will Deacon wrote: > On Mon, Aug 10, 2026 at 10:08:18AM -0300, 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 <[email protected]> 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/[email protected]/. > > > > > 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. > > To be clear: the only part I'm pushing back on is the realm-specific hack > to size the SWIOTLB area in arch/arm64/. Even with that hack applied, I > don't believe a one-size-fits-all value is going to work for everybody, > so it was really great to see that special-case removed in: > > https://lore.kernel.org/all/[email protected]/
We had Mostafa's patch here: https://lore.kernel.org/all/[email protected]/ adding the same swiotlb buffer allocation for pKVM guests and realms. Aneesh's cleanup of SWIOTLB_FORCE inadvertently removed the 'else' clause and limited_addressing is now always checked, meaning that we get size limiting irrespective of whether we still need a default swiotlb. Normally I would consider this a regression on top of mainline and Mostafa's patches rather than a cleanup. However, if the default size of the bounce buffer was never sufficient for realms, as you said, we may need something better here anyway, so not worth fixing. I can see x86 in mem_encrypt_setup_arch() adjusting the bounce buffer to 6% of the guest memory. We could make up similar logic for arm64 or we could just rely on CONFIG_SWIOTLB_DYNAMIC (default off currently for arm64 I think). -- Catalin
