Re: [PATCH 1/9] AMD/IOMMU: redo awaiting of command completion

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.06.2021 12:36, Andrew Cooper wrote:
> On 09/06/2021 10:26, Jan Beulich wrote:
>> @@ -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.

Long ago I had an item on my todo list to get this cleaned up. But
it never really having made it up high enough, I dropped it at
some point, in the hope that we'd manage to get this sorted while
re-writing code step by step.

> 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.

I don't think I can spot a single instance of such. Some work was
done to introduce (mainly bitfield) structs, but this surely goes
too far for the change at hand. I can spot two instances using
MASK_INSR(), so I can see two consistent ways of doing what you
ask for:

    cmd[0] = addr | MASK_INSR(IOMMU_CONTROL_ENABLED,
                              IOMMU_COMP_WAIT_S_FLAG_MASK);

keeping the name as *_MASK (and I'd be open to replace
IOMMU_CONTROL_ENABLED by true) or

    cmd[0] = addr | IOMMU_COMP_WAIT_S_FLAG;

i.e. dropping _MASK (but requiring adjustments elsewhere in the
code). Please let me know which one you'd prefer.

> 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.

Well, I can surely do so, but like this entire request of yours this
feels like scope creep - there was no intention here to do any
unrelated cleanup. And if I remove _S_ and _I_, then surely _F_
wants dropping as well, while IOMMU_COMP_WAIT_ADDR_*_SHIFT have a
use each in iommu_guest.c and hence need to stay for now.

> 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.

Can do (using IOMMU_CMD_OPCODE_MASK).

Jan