linux-kernel - Re: [PATCH v5 2/6] iommu/amd: Introduce helper function to update 256-bit DTE

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Message-ID: <CAFULd4b12ytimeSiePmwppyEjQHQMSikHuG2pO7DhxMxwu3z_Q@mail.gmail.com>
Date: Fri, 11 Oct 2024 13:05:20 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, joro@...tes.org, 
	robin.murphy@....com, vasant.hegde@....com, jgg@...dia.com, 
	kevin.tian@...el.com, jon.grimm@....com, santosh.shukla@....com, 
	pandoh@...gle.com, kumaranand@...gle.com
Subject: Re: [PATCH v5 2/6] iommu/amd: Introduce helper function to update
 256-bit DTE

On Fri, Oct 11, 2024 at 12:22 PM Suthikulpanit, Suravee
<suravee.suthikulpanit@....com> wrote:
>
> On 10/7/2024 9:42 PM, Uros Bizjak wrote:
> >
> >
> > On 7. 10. 24 06:13, Suravee Suthikulpanit wrote:
> >
> >> +
> >>
> >> /****************************************************************************
> >>    *
> >>    * Helper functions
> >>    *
> >>
> >> ****************************************************************************/
> >> +static void write_dte_upper128(struct dev_table_entry *ptr, struct
> >> dev_table_entry *new)
> >> +{
> >> +    struct dev_table_entry old = {};
> >> +
> >> +    do {
> >> +        old.data128[1] = ptr->data128[1];
> >> +        new->data[2] &= ~DTE_DATA2_INTR_MASK;
> >> +        new->data[2] |= old.data[2] & (DTE_DATA2_INTR_MASK |
> >> DTE_DATA2_RESV_MASK);
> >> +    } while (!try_cmpxchg128(&ptr->data128[1], &old.data128[1],
> >> new->data128[1]));
> >
> > Please note that try_cmpxchg inherently updates &old.data128[1] above on
> > failure. There is no need to update value again in the loop.
> >
> > Please also note that the value from ptr->data128[1] should be read
> > using READ_ONCE() to prevent compiler from merging, refetching or
> > reordering the read. Currently, there is no READ_ONCE() implemented for
> > __int128, so something like the attached patch should be used.
>
> Thanks for pointing this out. I will introduce the attached patch
> separately in this series on your behalf as author/sign-off, and review
> the current code to properly use the READ_ONCE().

FTR, for the mentioned patch:

Signed-off-by: Uros Bizjak <ubizjak@...il.com>

for Co-authored-by: tag.

Thanks,
Uros.