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

[Suthikulpanit, Suravee]

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().

Thanks,
Suravee

> Based on the above, the loop should be rewritten as:
>
>      old.data128[1] = READ_ONCE(ptr->data128[1]);
>      do {
>          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]));
>
> > +}
> > +
> > +static void write_dte_lower128(struct dev_table_entry *ptr, struct 
> > dev_table_entry *new)
> > +{
> > +    struct dev_table_entry old = {};
> > +
> > +    /*
> > +     * Need to preserve DTE[96:106], which can be set by information 
> > in IVRS table.
> > +     * See set_dev_entry_from_acpi().
> > +     */
> > +    new->data[1] |= ptr->data[1] & DTE_FLAG_MASK;

> > +
> > +    do {
> > +        old.data128[0] = ptr->data128[0];
> > +    } while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], 
> > new->data128[0]));
>
> And this one as:
>
>      old.data128[0] = READ_ONCE(ptr->data128[0]);
>      do {
>      } while (!try_cmpxchg128(&ptr->data128[0], &old.data128[0], 
> new->data128[0]));
>
> Best regards,
> Uros.