Re: [PATCH 1/1] iommu/amd: Preserve default DTE fields when updating Host Page Table Root

[Arun Kodilkar, Sairaj]

On 1/7/2025 5:45 PM, Vasant Hegde wrote:
Hi vasant,
> On 1/7/2025 12:44 AM, Alejandro Jimenez wrote:
> > When updating the page table root field on the DTE, avoid overwriting any
> > bits that are already set. The earlier call to make_clear_dte() writes
> > default values that all DTEs must have set (currently DTE[V]), and those
> > must be preserved.
> >
> > Currently this doesn't cause problems since the page table root update is
> > the first field that is set after make_clear_dte() is called, and
> > DTE_FLAG_V is set again later along with the permission bits (IR/IW).
> > Remove this redundant assignment too.
>
> Good catch!
>
> > > Fixes: fd5dff9de4be ("iommu/amd: Modify set_dte_entry() to use 256-bit DTE
> helpers")
> > Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@xxxxxxxxxx>
> > ---
> >   drivers/iommu/amd/iommu.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> > index 20177e18eb0d..a8c17b602c95 100644
> > --- a/drivers/iommu/amd/iommu.c
> > +++ b/drivers/iommu/amd/iommu.c
> > @@ -2052,12 +2052,12 @@ static void set_dte_entry(struct amd_iommu *iommu,
> >   	make_clear_dte(dev_data, dte, &new);
>
> This fix is fine. But with all changes we have done in this code does, do we
> really need make_clear_dte()?
>
> How about removing `make_clear_dte()` and moving DTE_FLAG_V to
> write_dte_lower128() ?

I dont think moving DTE_FLAG_V to write_dte_lower128() is a good
idea, because the order in which the function update_dte256()
assigns upper and lower 128 bits depends on this flag.

Regards
Sairaj Kodilkar

> -Vasant