RE: [PATCH V2 1/6] Swiotlb: Add Swiotlb bounce buffer remap function for HV IVM

["Michael Kelley (LINUX)" <mikelley@xxxxxxxxxxxxx>]

From: Tianyu Lan <ltykernel@xxxxxxxxx> Sent: Tuesday, November 23, 2021 6:31 AM
> 
> In Isolation VM with AMD SEV, bounce buffer needs to be accessed via
> extra address space which is above shared_gpa_boundary (E.G 39 bit
> address line) reported by Hyper-V CPUID ISOLATION_CONFIG. The access
> physical address will be original physical address + shared_gpa_boundary.
> The shared_gpa_boundary in the AMD SEV SNP spec is called virtual top of
> memory(vTOM). Memory addresses below vTOM are automatically treated as
> private while memory above vTOM is treated as shared.
> 
> Expose swiotlb_unencrypted_base for platforms to set unencrypted
> memory base offset and platform calls swiotlb_update_mem_attributes()
> to remap swiotlb mem to unencrypted address space. memremap() can
> not be called in the early stage and so put remapping code into
> swiotlb_update_mem_attributes(). Store remap address and use it to copy
> data from/to swiotlb bounce buffer.
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> ---
> Change since v1:
>       * Rework comment in the swiotlb_init_io_tlb_mem()
>       * Make swiotlb_init_io_tlb_mem() back to return void.
> ---
>  include/linux/swiotlb.h |  6 +++++
>  kernel/dma/swiotlb.c    | 53 +++++++++++++++++++++++++++++++++++++----
>  2 files changed, 54 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 569272871375..f6c3638255d5 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -73,6 +73,9 @@ extern enum swiotlb_force swiotlb_force;
>   * @end:     The end address of the swiotlb memory pool. Used to do a quick
>   *           range check to see if the memory was in fact allocated by this
>   *           API.
> + * @vaddr:   The vaddr of the swiotlb memory pool. The swiotlb memory pool
> + *           may be remapped in the memory encrypted case and store virtual
> + *           address for bounce buffer operation.
>   * @nslabs:  The number of IO TLB blocks (in groups of 64) between @start and
>   *           @end. For default swiotlb, this is command line adjustable via
>   *           setup_io_tlb_npages.
> @@ -92,6 +95,7 @@ extern enum swiotlb_force swiotlb_force;
>  struct io_tlb_mem {
>       phys_addr_t start;
>       phys_addr_t end;
> +     void *vaddr;
>       unsigned long nslabs;
>       unsigned long used;
>       unsigned int index;
> @@ -186,4 +190,6 @@ static inline bool is_swiotlb_for_alloc(struct device 
> *dev)
>  }
>  #endif /* CONFIG_DMA_RESTRICTED_POOL */
> 
> +extern phys_addr_t swiotlb_unencrypted_base;
> +
>  #endif /* __LINUX_SWIOTLB_H */
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 8e840fbbed7c..c303fdeba82f 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -50,6 +50,7 @@
>  #include <asm/io.h>
>  #include <asm/dma.h>
> 
> +#include <linux/io.h>
>  #include <linux/init.h>
>  #include <linux/memblock.h>
>  #include <linux/iommu-helper.h>
> @@ -72,6 +73,8 @@ enum swiotlb_force swiotlb_force;
> 
>  struct io_tlb_mem io_tlb_default_mem;
> 
> +phys_addr_t swiotlb_unencrypted_base;
> +
>  /*
>   * Max segment that we can provide which (if pages are contingous) will
>   * not be bounced (unless SWIOTLB_FORCE is set).
> @@ -155,6 +158,31 @@ static inline unsigned long nr_slots(u64 val)
>       return DIV_ROUND_UP(val, IO_TLB_SIZE);
>  }
> 
> +/*
> + * Remap swioltb memory in the unencrypted physical address space
> + * when swiotlb_unencrypted_base is set. (e.g. for Hyper-V AMD SEV-SNP
> + * Isolation VMs).
> + */
> +void *swiotlb_mem_remap(struct io_tlb_mem *mem, unsigned long bytes)
> +{
> +     void *vaddr;
> +
> +     if (swiotlb_unencrypted_base) {
> +             phys_addr_t paddr = mem->start + swiotlb_unencrypted_base;
> +
> +             vaddr = memremap(paddr, bytes, MEMREMAP_WB);
> +             if (!vaddr) {
> +                     pr_err("Failed to map the unencrypted memory %llx size 
> %lx.\n",
> +                            paddr, bytes);
> +                     return NULL;
> +             }
> +
> +             return vaddr;
> +     }
> +
> +     return phys_to_virt(mem->start);
> +}
> +
>  /*
>   * Early SWIOTLB allocation may be too early to allow an architecture to
>   * perform the desired operations.  This function allows the architecture to
> @@ -172,7 +200,14 @@ void __init swiotlb_update_mem_attributes(void)
>       vaddr = phys_to_virt(mem->start);
>       bytes = PAGE_ALIGN(mem->nslabs << IO_TLB_SHIFT);
>       set_memory_decrypted((unsigned long)vaddr, bytes >> PAGE_SHIFT);
> -     memset(vaddr, 0, bytes);
> +
> +     mem->vaddr = swiotlb_mem_remap(mem, bytes);
> +     if (!mem->vaddr) {
> +             pr_err("Fail to remap swiotlb mem.\n");
> +             return;
> +     }
> +
> +     memset(mem->vaddr, 0, bytes);
>  }

In the error case, do you want to leave mem->vaddr as NULL?  Or is it
better to leave it as the virtual address of mem-start?  Your code leaves it
as NULL.

The interaction between swiotlb_update_mem_attributes() and the helper
function swiotlb_memo_remap() seems kind of clunky.  phys_to_virt() gets called
twice, for example, and two error messages are printed.  The code would be
more straightforward by just putting the helper function inline:

mem->vaddr = phys_to_virt(mem->start);
bytes = PAGE_ALIGN(mem->nslabs << IO_TLB_SHIFT);
set_memory_decrypted((unsigned long)(mem->vaddr), bytes >> PAGE_SHIFT);

if (swiotlb_unencrypted_base) {
        phys_addr_t paddr = mem->start + swiotlb_unencrypted_base;

        mem->vaddr = memremap(paddr, bytes, MEMREMAP_WB);
        if (!mem->vaddr) {
                pr_err("Failed to map the unencrypted memory %llx size %lx.\n",
                               paddr, bytes);
                return;
        }
}

memset(mem->vaddr, 0, bytes);

(This version also leaves mem->vaddr as NULL in the error case.)

> 
>  static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t 
> start,
> @@ -196,7 +231,18 @@ static void swiotlb_init_io_tlb_mem(struct io_tlb_mem 
> *mem, phys_addr_t start,
>               mem->slots[i].orig_addr = INVALID_PHYS_ADDR;
>               mem->slots[i].alloc_size = 0;
>       }
> +
> +     /*
> +      * If swiotlb_unencrypted_base is set, the bounce buffer memory will
> +      * be remapped and cleared in swiotlb_update_mem_attributes.
> +      */
> +     if (swiotlb_unencrypted_base)
> +             return;
> +
> +     set_memory_decrypted((unsigned long)vaddr, bytes >> PAGE_SHIFT);

Prior to this patch, and here in the new version as well, the return value from
set_memory_decrypted() is ignored in several places in this file.  As previously
discussed, swiotlb_init_io_tlb_mem() is a void function, so there's no place to
return an error. Is that OK?

>       memset(vaddr, 0, bytes);
> +     mem->vaddr = vaddr;
> +     return;
>  }
> 
>  int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int 
> verbose)
> @@ -318,7 +364,6 @@ swiotlb_late_init_with_tbl(char *tlb, unsigned long 
> nslabs)
>       if (!mem->slots)
>               return -ENOMEM;
> 
> -     set_memory_decrypted((unsigned long)tlb, bytes >> PAGE_SHIFT);
>       swiotlb_init_io_tlb_mem(mem, virt_to_phys(tlb), nslabs, true);
> 
>       swiotlb_print_info();
> @@ -371,7 +416,7 @@ static void swiotlb_bounce(struct device *dev, 
> phys_addr_t tlb_addr, size_t size
>       phys_addr_t orig_addr = mem->slots[index].orig_addr;
>       size_t alloc_size = mem->slots[index].alloc_size;
>       unsigned long pfn = PFN_DOWN(orig_addr);
> -     unsigned char *vaddr = phys_to_virt(tlb_addr);
> +     unsigned char *vaddr = mem->vaddr + tlb_addr - mem->start;
>       unsigned int tlb_offset, orig_addr_offset;
> 
>       if (orig_addr == INVALID_PHYS_ADDR)
> @@ -806,8 +851,6 @@ static int rmem_swiotlb_device_init(struct reserved_mem 
> *rmem,
>                       return -ENOMEM;
>               }
> 
> -             set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
> -                                  rmem->size >> PAGE_SHIFT);
>               swiotlb_init_io_tlb_mem(mem, rmem->base, nslabs, false);
>               mem->force_bounce = true;
>               mem->for_alloc = true;
> --
> 2.25.1