Re: [RFC PATCH] dma: swiotlb: Size shared default pools for memory encryption
From: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Date: 2026-08-12 11:31:53
Also in:
linux-arm-kernel, linux-coco, linux-iommu, linux-s390, lkml
Catalin Marinas [off-list ref] writes:
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.
I have a follow-up cleanup patch where I switch the swiotlb initialization reason to a flag value. With that, we now have SWIOTLB_INIT_CC_SHARED. The challenge is that it is still not clear when pKVM would want to use an unencrypted swiotlb pool for DMA bouncing. I would expect pKVM to have some additional condition based on which it chooses either the restricted-dma-pool or the default swiotlb pool. Using the presence of a restricted-dma-pool to decide whether the default swiotlb pool should be unencrypted is one option, but I am not sure that is the right abstraction. modified arch/arm64/mm/init.c
@@ -342,7 +342,7 @@ void __init arch_mm_preinit(void) bool cc_guest = is_realm_world(); if (cc_guest) - swiotlb_mark_default_cc_shared(); + flags |= SWIOTLB_INIT_CC_SHARED; else if (max_pfn > PFN_DOWN(arm64_dma_phys_limit)) flags |= SWIOTLB_INIT_ADDRESSING_LIMIT;
modified arch/powerpc/mm/mem.c
@@ -289,9 +289,8 @@ void __init arch_mm_preinit(void) #ifdef CONFIG_SWIOTLB if (is_secure_guest()) { /* The host can access DMA buffers only through the SWIOTLB. */ - swiotlb_mark_default_cc_shared(); ppc_swiotlb_enable = 1; - ppc_swiotlb_flags |= SWIOTLB_ANY; + ppc_swiotlb_flags |= SWIOTLB_INIT_CC_SHARED | SWIOTLB_ANY; } /*
modified arch/s390/mm/init.c
@@ -163,12 +163,10 @@ static void __init pv_init(void) if (!is_prot_virt_guest()) return; - swiotlb_mark_default_cc_shared(); - virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc); /* make sure bounce buffers are shared */ - swiotlb_init(SWIOTLB_VERBOSE); + swiotlb_init(SWIOTLB_VERBOSE | SWIOTLB_INIT_CC_SHARED); swiotlb_update_mem_attributes(); }
modified arch/x86/mm/mem_encrypt.c
@@ -14,6 +14,7 @@ #include <linux/mem_encrypt.h> #include <linux/virtio_anchor.h> +#include <asm/iommu.h> #include <asm/sev.h> /* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
@@ -109,7 +110,7 @@ void __init mem_encrypt_setup_arch(void) snp_fixup_e820_tables(); if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) - swiotlb_mark_default_cc_shared(); + x86_swiotlb_flags |= SWIOTLB_INIT_CC_SHARED; if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) return;
modified include/linux/swiotlb.h
@@ -18,6 +18,8 @@ struct scatterlist; #define SWIOTLB_ANY (1 << 1) /* allow any memory for the buffer */ /* Initialize a default-sized pool for devices with limited DMA addressing. */ #define SWIOTLB_INIT_ADDRESSING_LIMIT (1 << 2) +/* Initialize a shared default pool for confidential-computing systems. */ +#define SWIOTLB_INIT_CC_SHARED (1 << 3) /* * Maximum allowable number of contiguous slabs to map,
@@ -191,7 +193,6 @@ void swiotlb_dev_init(struct device *dev); size_t swiotlb_max_mapping_size(struct device *dev); bool is_swiotlb_allocated(void); bool is_swiotlb_active(struct device *dev); -void __init swiotlb_mark_default_cc_shared(void); void __init swiotlb_adjust_size(unsigned long size); phys_addr_t default_swiotlb_base(void); phys_addr_t default_swiotlb_limit(void);
@@ -231,10 +232,6 @@ static inline bool is_swiotlb_active(struct device *dev) return false; } -static inline void swiotlb_mark_default_cc_shared(void) -{ -} - static inline void swiotlb_adjust_size(unsigned long size) { }
modified kernel/dma/swiotlb.c
@@ -373,11 +373,6 @@ static void __init *swiotlb_memblock_alloc(unsigned long nslabs, return tlb; } -void __init swiotlb_mark_default_cc_shared(void) -{ - io_tlb_default_mem.cc_shared = true; -} - static void __init swiotlb_adjust_cc_attributes(void) { unsigned long size;
@@ -429,7 +424,7 @@ static bool __init swiotlb_should_init(unsigned int flags) if (swiotlb_force_bounce) return true; - if (io_tlb_default_mem.cc_shared) + if (flags & SWIOTLB_INIT_CC_SHARED) return true; return false;
@@ -451,6 +446,9 @@ void __init swiotlb_init_remap(unsigned int flags, if (!swiotlb_should_init(flags)) return; + if (flags & SWIOTLB_INIT_CC_SHARED) + io_tlb_default_mem.cc_shared = true; + io_tlb_default_mem.force_bounce = swiotlb_force_bounce; #ifdef CONFIG_SWIOTLB_DYNAMIC