Re: [PATCH v2 5/5] iommu/amd: Use 128-bit cmpxchg in set_dte_irq_entry()

["Suthikulpanit, Suravee" <suravee.suthikulpanit@amd.com>]

Hi

On 8/30/2024 2:40 AM, Jason Gunthorpe wrote:
> On Thu, Aug 29, 2024 at 06:07:26PM +0000, Suravee Suthikulpanit wrote:
>> Interrupt-remapping-related fields are in the top 128-bit of the Device
>> Table Entry (DTE), which should be updated using 128-bit write based on the
>> AMD I/O Virtualization Techonology (IOMMU) Specification.
>>
>> Therefore, modify set_dte_irq_entry() to use 128-bit cmpxchg. Also, use
>> struct dev_data->dte_sem to synchronize DTE access.
>>
>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>> ---
>>   drivers/iommu/amd/iommu.c | 25 ++++++++++++++++---------
>>   1 file changed, 16 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>> index a24986c2478b..4eb53bd40487 100644
>> --- a/drivers/iommu/amd/iommu.c
>> +++ b/drivers/iommu/amd/iommu.c
>> @@ -3020,17 +3020,24 @@ static void iommu_flush_irt_and_complete(struct amd_iommu *iommu, u16 devid)
>>   static void set_dte_irq_entry(struct amd_iommu *iommu, u16 devid,
>>   			      struct irq_remap_table *table)
>>   {
>> -	u64 dte;
>> -	struct dev_table_entry *dev_table = get_dev_table(iommu);
>> +	u128 new, old;
>> +	struct dev_table_entry *dte = &get_dev_table(iommu)[devid];
>> +	struct iommu_dev_data *dev_data = search_dev_data(iommu, devid);
>> +
>> +	if (dev_data)
>> +		down_write(&dev_data->dte_sem);
>> +
>> +	old = new = dte->data128[1];
>> +	new &= ~DTE_IRQ_PHYS_ADDR_MASK;
>> +	new |= iommu_virt_to_phys(table->table);
>> +	new |= DTE_IRQ_REMAP_INTCTL;
>> +	new |= DTE_INTTABLEN;
>> +	new |= DTE_IRQ_REMAP_ENABLE;
>>   
>> -	dte	= dev_table[devid].data[2];
>> -	dte	&= ~DTE_IRQ_PHYS_ADDR_MASK;
>> -	dte	|= iommu_virt_to_phys(table->table);
>> -	dte	|= DTE_IRQ_REMAP_INTCTL;
>> -	dte	|= DTE_INTTABLEN;
>> -	dte	|= DTE_IRQ_REMAP_ENABLE;
>> +	WARN_ON(!try_cmpxchg128(&dte->data128[1], &old, new));
> 
> This probably doesn't need cmpxchg because it is only touching 64 bit
> quanta [2], just a lock is good enough and avoids the "what to do if
> cmpxchg fails" question.

Okay

>> -	dev_table[devid].data[2] = dte;
> 
> But this should be
> 
>    WRITE_ONCE(dev_table[devid].data[2], dte);
> 
> Beaucse it is writing to memory read back by HW.
> 
> Required for all the DTE touches everywhere.

Good point.

Thanks,
Suravee