> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.coop...@citrix.com]
> Sent: 24 September 2018 11:55
> To: Xen-devel <xen-de...@lists.xen.org>
> Cc: Andrew Cooper <andrew.coop...@citrix.com>; Jan Beulich
> <jbeul...@suse.com>; Wei Liu <wei.l...@citrix.com>; Roger Pau Monne
> <roger....@citrix.com>; Paul Durrant <paul.durr...@citrix.com>; Suravee
> Suthikulpanit <suravee.suthikulpa...@amd.com>; Brian Woods
> <brian.wo...@amd.com>
> Subject: [PATCH 1/2] x86/SVM-IOMMU: Don't opencode memcpy() in
> queue_iommu_command()
>
> In practice, this allows the compiler to replace the loop with a pair of
> movs.
>
> No functional change.
Well there is a potential functional change...
>
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
> ---
> CC: Jan Beulich <jbeul...@suse.com>
> CC: Wei Liu <wei.l...@citrix.com>
> CC: Roger Pau Monné <roger....@citrix.com>
> CC: Paul Durrant <paul.durr...@citrix.com>
> CC: Suravee Suthikulpanit <suravee.suthikulpa...@amd.com>
> CC: Brian Woods <brian.wo...@amd.com>
> ---
> xen/drivers/passthrough/amd/iommu_cmd.c | 12 ++++--------
> xen/include/asm-x86/hvm/svm/amd-iommu-defs.h | 1 -
> 2 files changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/xen/drivers/passthrough/amd/iommu_cmd.c
> b/xen/drivers/passthrough/amd/iommu_cmd.c
> index 08247fa..c6c0b4f 100644
> --- a/xen/drivers/passthrough/amd/iommu_cmd.c
> +++ b/xen/drivers/passthrough/amd/iommu_cmd.c
> @@ -24,8 +24,7 @@
>
> static int queue_iommu_command(struct amd_iommu *iommu, u32 cmd[])
> {
> - u32 tail, head, *cmd_buffer;
> - int i;
> + uint32_t tail, head;
>
> tail = iommu->cmd_buffer.tail;
> if ( ++tail == iommu->cmd_buffer.entries )
> @@ -35,12 +34,9 @@ static int queue_iommu_command(struct amd_iommu *iommu,
> u32 cmd[])
> IOMMU_CMD_BUFFER_HEAD_OFFSET));
> if ( head != tail )
> {
> - cmd_buffer = (u32 *)(iommu->cmd_buffer.buffer +
> - (iommu->cmd_buffer.tail *
> - IOMMU_CMD_BUFFER_ENTRY_SIZE));
> -
> - for ( i = 0; i < IOMMU_CMD_BUFFER_U32_PER_ENTRY; i++ )
> - cmd_buffer[i] = cmd[i];
> + memcpy(iommu->cmd_buffer.buffer +
> + (iommu->cmd_buffer.tail * IOMMU_CMD_BUFFER_ENTRY_SIZE),
> + cmd, IOMMU_CMD_BUFFER_ENTRY_SIZE);
...since the built-in memcpy may not guarantee to the copy in 4 byte chunks in
ascending order. As long as that change poses no danger (and seeing as the code
contains no barrer-ing I'd assume that to be the case) then this change looks
fine to me (with the 'no functional change' comment removed).
Reviewed-by: Paul Durrant <paul.durr...@citrix.com>
>
> iommu->cmd_buffer.tail = tail;
> return 1;
> diff --git a/xen/include/asm-x86/hvm/svm/amd-iommu-defs.h
> b/xen/include/asm-x86/hvm/svm/amd-iommu-defs.h
> index c479f0b..1f19cd3 100644
> --- a/xen/include/asm-x86/hvm/svm/amd-iommu-defs.h
> +++ b/xen/include/asm-x86/hvm/svm/amd-iommu-defs.h
> @@ -187,7 +187,6 @@
>
> #define IOMMU_CMD_BUFFER_ENTRY_SIZE 16
> #define IOMMU_CMD_BUFFER_POWER_OF2_ENTRIES_PER_PAGE 8
> -#define IOMMU_CMD_BUFFER_U32_PER_ENTRY (IOMMU_CMD_BUFFER_ENTRY_SIZE /
> 4)
>
> #define IOMMU_CMD_OPCODE_MASK 0xF0000000
> #define IOMMU_CMD_OPCODE_SHIFT 28
> --
> 2.1.4
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