On 09.06.2021 12:36, Andrew Cooper wrote:
> On 09/06/2021 10:26, Jan Beulich wrote:
>> The present abuse of the completion interrupt does not only stand in the
>> way of, down the road, using it for its actual purpose, but also
>> requires holding the IOMMU lock while waiting for command completion,
>> limiting parallelism and keeping interrupts off for non-negligible
>> periods of time. Have the IOMMU do an ordinary memory write instead of
>> signaling an otherwise disabled interrupt (by just updating a status
>> register bit).
>>
>> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>> Reviewed-by: Paul Durrant <p...@xen.org>
>
> While I agree with the direction of the patch, some of the details could
> do with improvement.
>
>>
>> --- a/xen/drivers/passthrough/amd/iommu_cmd.c
>> +++ b/xen/drivers/passthrough/amd/iommu_cmd.c
>> @@ -20,6 +20,9 @@
>> #include "iommu.h"
>> #include "../ats.h"
>>
>> +#define CMD_COMPLETION_INIT 0
>> +#define CMD_COMPLETION_DONE 1
>> +
>> static void send_iommu_command(struct amd_iommu *iommu,
>> const uint32_t cmd[4])
>> {
>> @@ -49,28 +52,31 @@ static void send_iommu_command(struct am
>> static void flush_command_buffer(struct amd_iommu *iommu,
>> unsigned int timeout_base)
>> {
>> + static DEFINE_PER_CPU(uint64_t, poll_slot);
>> + uint64_t *this_poll_slot = &this_cpu(poll_slot);
>> + paddr_t addr = virt_to_maddr(this_poll_slot);
>> uint32_t cmd[4];
>> s_time_t start, timeout;
>> static unsigned int __read_mostly threshold = 1;
>>
>> - /* RW1C 'ComWaitInt' in status register */
>> - writel(IOMMU_STATUS_COMP_WAIT_INT,
>> - iommu->mmio_base + IOMMU_STATUS_MMIO_OFFSET);
>> -
>> - /* send an empty COMPLETION_WAIT command to flush command buffer */
>> - cmd[3] = cmd[2] = 0;
>> - set_field_in_reg_u32(IOMMU_CMD_COMPLETION_WAIT, 0,
>> + ACCESS_ONCE(*this_poll_slot) = CMD_COMPLETION_INIT;
>> +
>> + /* send a COMPLETION_WAIT command to flush command buffer */
>> + cmd[0] = addr;
>> + set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, cmd[0],
>> + IOMMU_COMP_WAIT_S_FLAG_MASK,
>> + IOMMU_COMP_WAIT_S_FLAG_SHIFT, &cmd[0]);
>
> set_field_in_reg_u32() is a disaster of a function - both in terms of
> semantics, and code gen - and needs to be purged from the code.
>
> It is a shame we don't have a real struct for objects in the command
> buffer, but in lieu of that, this is just
>
> cmd[0] = addr | IOMMU_COMP_WAIT_S_FLAG_MASK;
>
> which is the direction that previous cleanup has gone.
>
> There are no current users of IOMMU_COMP_WAIT_S_FLAG_SHIFT, and ...
>
>> + cmd[1] = addr >> 32;
>> + set_field_in_reg_u32(IOMMU_CMD_COMPLETION_WAIT, cmd[1],
>> IOMMU_CMD_OPCODE_MASK,
>> IOMMU_CMD_OPCODE_SHIFT, &cmd[1]);
>> - set_field_in_reg_u32(IOMMU_CONTROL_ENABLED, 0,
>> - IOMMU_COMP_WAIT_I_FLAG_MASK,
>> - IOMMU_COMP_WAIT_I_FLAG_SHIFT, &cmd[0]);
>
> ... this drops the final use of IOMMU_COMP_WAIT_I_FLAG_SHIFT, so both
> should be dropped.
>
> As for IOMMU_CMD_OPCODE_SHIFT, that can't be dropped yet, but it would
> still be better to use
>
> cmd[1] = (addr >> 32) | MASK_INSR(IOMMU_CMD_COMPLETION_WAIT,
> IOMMU_CMD_COMPLETION_WAIT);
>
> in the short term.
Okay, this conversion has indeed saved a single
and $0x0FFFFFFF, %eax
But we're down by two set_field_in_reg_u32() now; only some 30 left.
Jan
