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Re: [Xen-devel] [PATCH 1/2] x86/SVM-IOMMU: Don't opencode memcpy() in queue_iommu_command()
- To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 24 Sep 2018 13:05:53 +0100
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- Cc: Brian Woods <brian.woods@xxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 24 Sep 2018 12:06:19 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 24/09/18 12:59, Paul Durrant wrote:
>> -----Original Message-----
>> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
>> Sent: 24 September 2018 11:55
>> To: Xen-devel <xen-devel@xxxxxxxxxxxxx>
>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich
>> <JBeulich@xxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Roger Pau Monne
>> <roger.pau@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Suravee
>> Suthikulpanit <suravee.suthikulpanit@xxxxxxx>; Brian Woods
>> <brian.woods@xxxxxxx>
>> 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.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Paul Durrant <paul.durrant@xxxxxxxxxx>
>> CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
>> CC: Brian Woods <brian.woods@xxxxxxx>
>> ---
>> 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.
"No functional change" != "The binary is identical".
The functionality of queue_iommu_command() does not change, even if it's
code generation does. It is just copying bytes into a shared ring,
bounded later by updating the tail pointer.
~Andrew
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