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SubjectRE: [PATCH v4 04/12] iommu/vt-d: Add 256-bit invalidation descriptor support
Date

> From: Lu Baolu [mailto:baolu.lu@linux.intel.com]
> Sent: Thursday, November 8, 2018 1:25 PM
> Subject: Re: [PATCH v4 04/12] iommu/vt-d: Add 256-bit invalidation descriptor
> support
>
> Hi,
>
> On 11/8/18 11:49 AM, Liu, Yi L wrote:
> > Hi,
> >
> >> From: Lu Baolu [mailto:baolu.lu@linux.intel.com]
> >> Sent: Thursday, November 8, 2018 10:17 AM
> >> Subject: Re: [PATCH v4 04/12] iommu/vt-d: Add 256-bit invalidation
> >> descriptor support
> >>
> >> Hi Yi,
> >>
> >> On 11/7/18 2:07 PM, Liu, Yi L wrote:
> >>> Hi Baolu,
> >>>
> >>>> From: Lu Baolu [mailto:baolu.lu@linux.intel.com]
> >>>> Sent: Monday, November 5, 2018 1:32 PM
> >>>
> >>> [...]
> >>>
> >>>> ---
> >>>> drivers/iommu/dmar.c | 83 +++++++++++++++++++----------
> >>>> drivers/iommu/intel-svm.c | 76 ++++++++++++++++----------
> >>>> drivers/iommu/intel_irq_remapping.c | 6 ++-
> >>>> include/linux/intel-iommu.h | 9 +++-
> >>>> 4 files changed, 115 insertions(+), 59 deletions(-)
> >>>>
> >>>> diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c index
> >>>> d9c748b6f9e4..ec10427b98ac 100644
> >>>> --- a/drivers/iommu/dmar.c
> >>>> +++ b/drivers/iommu/dmar.c
> >>>> @@ -1160,6 +1160,7 @@ static int qi_check_fault(struct intel_iommu
> >>>> *iommu, int
> >>>> index)
> >>>> int head, tail;
> >>>> struct q_inval *qi = iommu->qi;
> >>>> int wait_index = (index + 1) % QI_LENGTH;
> >>>> + int shift = qi_shift(iommu);
> >>>>
> >>>> if (qi->desc_status[wait_index] == QI_ABORT)
> >>>> return -EAGAIN;
> >>>> @@ -1173,13 +1174,15 @@ static int qi_check_fault(struct
> >>>> intel_iommu *iommu, int index)
> >>>> */
> >>>> if (fault & DMA_FSTS_IQE) {
> >>>> head = readl(iommu->reg + DMAR_IQH_REG);
> >>>> - if ((head >> DMAR_IQ_SHIFT) == index) {
> >>>> + if ((head >> shift) == index) {
> >>>> + struct qi_desc *desc = qi->desc + head;
> >>>> +
> >>>> pr_err("VT-d detected invalid descriptor: "
> >>>> "low=%llx, high=%llx\n",
> >>>> - (unsigned long long)qi->desc[index].low,
> >>>> - (unsigned long long)qi->desc[index].high);
> >>>> - memcpy(&qi->desc[index], &qi->desc[wait_index],
> >>>> - sizeof(struct qi_desc));
> >>>> + (unsigned long long)desc->qw0,
> >>>> + (unsigned long long)desc->qw1);
> >>>
> >>> Still missing qw2 and qw3. May make the print differ based on if smts is configed.
> >>
> >> qw2 and qw3 are reserved from software point of view. We don't need
> >> to print it for information.
> >
> > But for Scalable mode, it should be valid?
>
> No. It's reserved for software.

No, I don’t think so. PRQ response would also be queued to hardware by QI. For such
QI descriptors, the high bits are not reserved.

> >>
> >>>
> >>>> + memcpy(desc, qi->desc + (wait_index << shift),
> >>>
> >>> Would "memcpy(desc, (unsigned long long) (qi->desc + (wait_index <<
> >>> shift)," be more safe?
> >>
> >> Can that be compiled? memcpy() requires a "const void *" for the second
> parameter.
> >> By the way, why it's safer with this casting?
> >
> > This is just an example. My point is the possibility that "qi->desc + (wait_index <<
> shift)"
> > would be treated as "qi->desc plus (wait_index <<
> > shift)*sizeof(*qi->desc)". Is it possible for kernel build?
>
> qi->desc is of type of "void *".

no, I don’t think so... Refer to the code below. Even it has no correctness issue her,
It's not due to qi->desc is "void *" type...

struct qi_desc {
- u64 low, high;
+ u64 qw0;
+ u64 qw1;
+ u64 qw2;
+ u64 qw3;
};

Regards,
Yi Liu
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