Catalin Marinas <[email protected]> writes: > On Tue, Aug 11, 2026 at 07:10:56PM +0530, Aneesh Kumar K.V (Arm) wrote: >> Systems with memory encryption use swiotlb to provide shared or >> unencrypted buffers for device DMA. Confidential guests may route all >> DMA through these buffers, while SME hosts use them for devices that >> cannot address encrypted memory. The default swiotlb pool can therefore >> be exhausted under I/O-intensive workloads. >> >> Let architectures mark the default swiotlb pool as shared before >> swiotlb_init(). > > I thought we wanted even this decision to be moved out of the arch code. >
Architectures may want to use an unencrypted swiotlb pool for different reasons, one of them being CC_ATTR_GUEST_MEM_ENCRYPT. x86 hosts also require unencrypted pool to support SME. We can cover both cases using CC_ATTR_MEM_ENCRYPT. However, pKVM does not want an unencrypted SWIOTLB pool. So I was thinking it would be much cleaner to let the architecture code drive that decision. > >> Move the existing x86 sizing policy into generic swiotlb >> code and add early pool marking for arm64 Realm guests, powerpc secure >> guests, s390 protected-virtualization guests, and x86 memory-encryption >> platforms. Use CC_ATTR_MEM_ENCRYPT on x86 to include host SME, whose >> swiotlb pool must also be decrypted for devices that cannot address >> encrypted memory. > > That's a functional change for x86. For now, I'd keep it to > CC_ATTR_GUEST_MEM_ENCRYPT. > > BTW, why does arm64 report CC_ATTR_MEM_ENCRYPT instead of the GUEST > option in realms? > >> Move the powerpc secure-guest swiotlb enablement before initialization >> so that the shared pool is allocated with the required flags. pKVM >> guests continue to use a restricted DMA pool instead of the default >> swiotlb pool. >> >> Signed-off-by: Aneesh Kumar K.V (Arm) <[email protected]> >> --- >> arch/arm64/mm/init.c | 6 ++- >> arch/powerpc/mm/mem.c | 7 +++ >> arch/powerpc/platforms/pseries/svm.c | 10 ---- >> arch/s390/mm/init.c | 2 + >> arch/x86/mm/mem_encrypt.c | 27 ++-------- >> include/linux/swiotlb.h | 7 ++- >> kernel/dma/swiotlb.c | 73 ++++++++++++++++++++++------ >> 7 files changed, 82 insertions(+), 50 deletions(-) >> >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c >> index e308a7cabd12..4c022e8aed43 100644 >> --- a/arch/arm64/mm/init.c >> +++ b/arch/arm64/mm/init.c >> @@ -338,8 +338,12 @@ void __init arch_setup_zero_pages(void) >> void __init arch_mm_preinit(void) >> { >> unsigned int flags = SWIOTLB_VERBOSE; >> + /* pKVM uses restricted-dma-pool */ >> + bool cc_guest = is_realm_world(); >> >> - if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> + if (cc_guest) { >> + swiotlb_mark_default_cc_shared(); >> + } else if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> /* >> * If no bouncing needed for ZONE_DMA, reduce the swiotlb >> * buffer for kmalloc() bouncing to 1MB per 1GB of RAM. > > With your other reworking, doesn't the core code know the swiotlb will > be shared? Can it not make the decision to resize at that point based > solely on CC_ATTR_GUEST_MEM_ENCRYPT? > > I also wonder whether we could address Will's pKVM request not to > allocate a bounce buffer once pKVM guests will start reporting > CC_ATTR_GUEST_MEM_ENCRYPT. Some simple heuristic: if a > restricted-dma-pool is advertised in DT (it will end up in > rmem_swiotlb_setup()), skip resizing the default swiotlb. It's not > perfect but the bounce buffer can be overridden on the command line. > For the same reason I mentioned above, architectures may have different reasons for setting cc_shared = true. IMHO, it is cleaner to let the architecture code make that decision before swiotlb_init(). -aneesh
