Re: [PATCH] iommu: amd: Fix IOMMU perf counter clobbering during init

[Shuah Khan]

On 1/21/20 8:32 AM, Shuah Khan wrote:
> On 1/20/20 7:10 PM, Suravee Suthikulpanit wrote:
> > On 1/17/2020 5:08 PM, Joerg Roedel wrote:
> > > Adding Suravee, who wrote the IOMMU Perf Counter code.
> > >
> > > On Tue, Jan 14, 2020 at 08:12:20AM -0700, Shuah Khan wrote:
> > > > init_iommu_perf_ctr() clobbers the register when it checks write access
> > > > to IOMMU perf counters and fails to restore when they are writable.
> > > >
> > > > Add save and restore to fix it.
> > > >
> > > > Signed-off-by: Shuah Khan<skhan@xxxxxxxxxxxxxxxxxxx>
> > > > ---
> > > > Â drivers/iommu/amd_iommu_init.c | 22 ++++++++++++++++------
> > > > Â 1 file changed, 16 insertions(+), 6 deletions(-)
> > > Suravee, can you please review this patch?
> >
> > This looks ok. Does this fix certain issues? Or is this just for sanity.
>
> I didn't notice any problems. Counters aren't writable on my system.
> However, it certainly looks like a bog since registers aren't restored
> like in other places in this file where such checks are done on other
> registers.
>
> I see 2 banks and 4 counters on my system. Is it sufficient to check
> the first bank and first counter? In other words, if the first one
> isn't writable, are all counters non-writable?
>
> Should we read the config first and then, try to see if any of the
> counters are writable? I have a patch that does that, I can send it
> out for review.
>
> > Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>

Joerg,

Please don't pull this in. I introduced a bug in this patch. It always
returns amd_iommu_pc_present false even when it can write to the
counters. My bad.

I will send v2.

thanks,
-- Shuah