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    Subject[RFC PATCH 2/2] dma-direct: Fix dma_direct_{alloc,free}() for Hyperv-V IVMs
    Date
    In Hyper-V AMD SEV-SNP Isolated VMs, the virtual address returned by
    dma_direct_alloc() must map above dma_unencrypted_base because the
    memory is shared with the hardware device and must not be encrypted.

    Modify dma_direct_alloc() to do the necessary remapping. In
    dma_direct_free(), use the (unmodified) DMA address to derive the
    original virtual address and re-encrypt the pages.

    Suggested-by: Michael Kelley <mikelley@microsoft.com>
    Co-developed-by: Dexuan Cui <decui@microsoft.com>
    Signed-off-by: Dexuan Cui <decui@microsoft.com>
    Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
    ---
    kernel/dma/direct.c | 30 +++++++++++++++++++++++++++++-
    1 file changed, 29 insertions(+), 1 deletion(-)

    diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
    index 06b2b901e37a3..c4ce277687a49 100644
    --- a/kernel/dma/direct.c
    +++ b/kernel/dma/direct.c
    @@ -13,6 +13,7 @@
    #include <linux/vmalloc.h>
    #include <linux/set_memory.h>
    #include <linux/slab.h>
    +#include <linux/io.h> /* for memremap() */
    #include "direct.h"

    /*
    @@ -305,6 +306,21 @@ void *dma_direct_alloc(struct device *dev, size_t size,
    ret = page_address(page);
    if (dma_set_decrypted(dev, ret, size))
    goto out_free_pages;
    +#ifdef CONFIG_HAS_IOMEM
    + /*
    + * Remap the pages in the unencrypted physical address space
    + * when dma_unencrypted_base is set (e.g., for Hyper-V AMD
    + * SEV-SNP isolated guests).
    + */
    + if (dma_unencrypted_base) {
    + phys_addr_t ret_pa = virt_to_phys(ret);
    +
    + ret_pa += dma_unencrypted_base;
    + ret = memremap(ret_pa, size, MEMREMAP_WB);
    + if (!ret)
    + goto out_encrypt_pages;
    + }
    +#endif
    }

    memset(ret, 0, size);
    @@ -360,11 +376,23 @@ void dma_direct_free(struct device *dev, size_t size,
    dma_free_from_pool(dev, cpu_addr, PAGE_ALIGN(size)))
    return;

    - if (is_vmalloc_addr(cpu_addr)) {
    + /*
    + * If dma_unencrypted_base is set, the virtual address returned by
    + * dma_direct_alloc() is in the vmalloc address range.
    + */
    + if (!dma_unencrypted_base && is_vmalloc_addr(cpu_addr)) {
    vunmap(cpu_addr);
    } else {
    if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_CLEAR_UNCACHED))
    arch_dma_clear_uncached(cpu_addr, size);
    +#ifdef CONFIG_HAS_IOMEM
    + if (dma_unencrypted_base) {
    + memunmap(cpu_addr);
    + /* re-encrypt the pages using the original address */
    + cpu_addr = page_address(pfn_to_page(PHYS_PFN(
    + dma_to_phys(dev, dma_addr))));
    + }
    +#endif
    if (dma_set_encrypted(dev, cpu_addr, size))
    return;
    }
    --
    2.25.1
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