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SubjectRe: [PATCH 2/2] iommu: amd: Use cmpxchg_double() when updating 128-bit IRTE
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Hi,

I'll send out V2 with fixes to the review comments below ...

On 9/2/20 10:26 PM, Joao Martins wrote:
> On 9/2/20 5:51 AM, Suravee Suthikulpanit wrote:
>> When using 128-bit interrupt-remapping table entry (IRTE) (a.k.a GA mode),
>> current driver disables interrupt remapping when it updates the IRTE
>> so that the upper and lower 64-bit values can be updated safely.
>>
>> However, this creates a small window, where the interrupt could
>> arrive and result in IO_PAGE_FAULT (for interrupt) as shown below.
>>
>> IOMMU Driver Device IRQ
>> ============ ===========
>> irte.RemapEn=0
>> ...
>> change IRTE IRQ from device ==> IO_PAGE_FAULT !!
>> ...
>> irte.RemapEn=1
>>
>> This scenario has been observed when changing irq affinity on a system
>> running I/O-intensive workload, in which the destination APIC ID
>> in the IRTE is updated.
>>
>> Instead, use cmpxchg_double() to update the 128-bit IRTE at once without
>> disabling the interrupt remapping. However, this means several features,
>> which require GA (128-bit IRTE) support will also be affected if cmpxchg16b
>> is not supported (which is unprecedented for AMD processors w/ IOMMU).
>>
> Probably requires:
>
> Fixes: 880ac60e2538 ("iommu/amd: Introduce interrupt remapping ops structure")
>

Yes, I will include this in V2.

>
>> Reported-by: Sean Osborne <sean.m.osborne@oracle.com>
>> Tested-by: Erik Rockstrom <erik.rockstrom@oracle.com>
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>
> With the comments below addressed, FWIW:
>
> Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
>
>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>> index c652f16eb702..ad30467f6930 100644
>> --- a/drivers/iommu/amd/init.c
>> +++ b/drivers/iommu/amd/init.c
>> @@ -1511,7 +1511,14 @@ static int __init init_iommu_one(struct amd_iommu *iommu, struct ivhd_header *h)
>> iommu->mmio_phys_end = MMIO_REG_END_OFFSET;
>> else
>> iommu->mmio_phys_end = MMIO_CNTR_CONF_OFFSET;
>> - if (((h->efr_attr & (0x1 << IOMMU_FEAT_GASUP_SHIFT)) == 0))
>> +
>> + /*
>> + * Note: GA (128-bit IRTE) mode requires cmpxchg16b supports.
>> + * GAM also requires GA mode. Therefore, we need to
>> + * check cmbxchg16b support before enabling it.
>> + */
>
> s/cmbxchg16b/cmpxchg16b
>
>> + if (!boot_cpu_has(X86_FEATURE_CX16) ||
>> + ((h->efr_attr & (0x1 << IOMMU_FEAT_GASUP_SHIFT)) == 0))
>> amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_LEGACY;
>> break;
>> case 0x11:
>> @@ -1520,8 +1527,18 @@ static int __init init_iommu_one(struct amd_iommu *iommu, struct ivhd_header *h)
>> iommu->mmio_phys_end = MMIO_REG_END_OFFSET;
>> else
>> iommu->mmio_phys_end = MMIO_CNTR_CONF_OFFSET;
>> - if (((h->efr_reg & (0x1 << IOMMU_EFR_GASUP_SHIFT)) == 0))
>> +
>> + /*
>> + * Note: GA (128-bit IRTE) mode requires cmpxchg16b supports.
>> + * XT, GAM also requires GA mode. Therefore, we need to
>> + * check cmbxchg16b support before enabling them.
>
> s/cmbxchg16b/cmpxchg16b
>
>> + */
>> + if (boot_cpu_has(X86_FEATURE_CX16) ||
>
> You probably want !boot_cpu_has(X86_FEATURE_CX16) ?

.... Ah, sorry!! I forgot to change it back after testing for the negative case. Thank you for catching this.

Suravee

>
>> + ((h->efr_reg & (0x1 << IOMMU_EFR_GASUP_SHIFT)) == 0)) {
>> amd_iommu_guest_ir = AMD_IOMMU_GUEST_IR_LEGACY;
>> + break;
>> + }
>> +
>> /*
>> * Note: Since iommu_update_intcapxt() leverages
>> * the IOMMU MMIO access to MSI capability block registers

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