On Thu, Sep 24, 2026 at 11:37:54AM +0530, Aneesh Kumar K.V (Arm) wrote: > Memory-encrypted guests use shared or unencrypted memory for DMA and may > route all DMA through SWIOTLB. The default pool can therefore be too > small for I/O-intensive workloads. > > Move the existing x86 guest-sizing policy into the SWIOTLB core. For > SWIOTLB_POOL_CC_GUEST, size the pool to 6% of guest memory, clamped > between the normal default and 1 GiB. Preserve an explicit swiotlb= > size. > > Host memory encryption still selects a normal-sized shared pool and > does not use the guest-sizing policy. > > A restricted DMA pool already provides shared bounce buffers for its > devices. Record its presence during reserved-memory initialization and > do not select the confidential-guest default-pool policy solely because > guest memory encryption is active. > > Reviewed-by: Catalin Marinas <[email protected]> > Cc: Dave Hansen <[email protected]> > Cc: Andy Lutomirski <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Borislav Petkov <[email protected]> > Cc: [email protected] > Cc: "H. Peter Anvin" <[email protected]> > Cc: Marek Szyprowski <[email protected]> > Cc: Robin Murphy <[email protected]> > Signed-off-by: Aneesh Kumar K.V (Arm) <[email protected]> > --- > arch/x86/mm/mem_encrypt.c | 24 ------------------------ > kernel/dma/swiotlb.c | 27 +++++++++++++++++++++++++-- > 2 files changed, 25 insertions(+), 26 deletions(-) > > diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c > index 95bae74fdab2..912f22ca838f 100644 > --- a/arch/x86/mm/mem_encrypt.c > +++ b/arch/x86/mm/mem_encrypt.c > @@ -101,9 +101,6 @@ void __init mem_encrypt_init(void) > > void __init mem_encrypt_setup_arch(void) > { > - phys_addr_t total_mem = memblock_phys_mem_size(); > - unsigned long size; > - > /* > * Do RMP table fixups after the e820 tables have been setup by > * e820__memory_setup(). > @@ -114,27 +111,6 @@ void __init mem_encrypt_setup_arch(void) > if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) > return; > > - /* > - * For SEV and TDX, all DMA has to occur via shared/unencrypted pages. > - * Kernel uses SWIOTLB to make this happen without changing device > - * drivers. However, depending on the workload being run, the > - * default 64MB of SWIOTLB may not be enough and SWIOTLB may > - * run out of buffers for DMA, resulting in I/O errors and/or > - * performance degradation especially with high I/O workloads. > - * > - * Adjust the default size of SWIOTLB using a percentage of guest > - * memory for SWIOTLB buffers. Also, as the SWIOTLB bounce buffer > - * memory is allocated from low memory, ensure that the adjusted size > - * is within the limits of low available memory. > - * > - * The percentage of guest memory used here for SWIOTLB buffers > - * is more of an approximation of the static adjustment which > - * 64MB for <1G, and ~128M to 256M for 1G-to-4G, i.e., the 6% > - */ > - size = total_mem * 6 / 100; > - size = clamp_val(size, IO_TLB_DEFAULT_SIZE, SZ_1G); > - swiotlb_adjust_size(size); > - > /* Set restricted memory access for virtio. */ > virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc); > } > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c > index 75ee63cd354e..e626cca63ad1 100644 > --- a/kernel/dma/swiotlb.c > +++ b/kernel/dma/swiotlb.c > @@ -33,6 +33,7 @@ > #include <linux/kmsan-checks.h> > #include <linux/iommu-helper.h> > #include <linux/init.h> > +#include <linux/math64.h> > #include <linux/memblock.h> > #include <linux/mm.h> > #include <linux/pfn.h> > @@ -80,6 +81,7 @@ struct io_tlb_slot { > > static bool swiotlb_force_bounce; > static bool swiotlb_force_disable; > +static bool restricted_dma_pool_present __initdata; > > enum swiotlb_pool_policy { > SWIOTLB_POOL_NONE, > @@ -478,7 +480,25 @@ swiotlb_adjust_pool_size(enum swiotlb_pool_policy policy) > size = min(swiotlb_default_pool_size(), size); > break; > case SWIOTLB_POOL_CC_GUEST: > - return; > + /* > + * For SEV and TDX and CCA, all DMA has to occur via > + * shared/unencrypted pages. Kernel uses SWIOTLB to make this > + * happen without changing device drivers. However, depending on > + * the workload being run, the default 64MB of SWIOTLB may not > be > + * enough and SWIOTLB may run out of buffers for DMA, resulting > in > + * I/O errors and/or performance degradation especially with > high > + * I/O workloads. > + * > + * Adjust the default size of SWIOTLB using a percentage of > guest > + * memory for SWIOTLB buffers. > + * > + * The percentage of guest memory used here for SWIOTLB buffers > is > + * more of an approximation of the static adjustment which 64MB > for > + * <1G, and ~128M to 256M for 1G-to-4G, i.e., the 6% > + */ > + size = div_u64((u64)memblock_phys_mem_size() * 6, 100); > + size = clamp_val(size, IO_TLB_DEFAULT_SIZE, SZ_1G); > + break; > case SWIOTLB_POOL_NONE: > WARN(true, "Cannot adjust SWIOTLB size without a pool\n"); > return; > @@ -496,7 +516,8 @@ swiotlb_select_pool_policy(unsigned int flags) > if (swiotlb_force_disable) > return SWIOTLB_POOL_NONE; > > - if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) > + if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && > + !restricted_dma_pool_present) > return SWIOTLB_POOL_CC_GUEST;
I think this check on the restricted DMA pool is too general -- the pool could be tied to a specific DMA-capable peripheral and so treating its presence as a global property isn't right. Will
