Re: [PATCH v1] iommu/vt-d: Remove the redundant logic in first_level_by_default()

[Ethan Zhao]

On 5/29/2025 2:11 PM, Tian, Kevin wrote:
> > From: Ethan Zhao <haifeng.zhao@xxxxxxxxxxxxxxx>
> > Sent: Thursday, May 29, 2025 1:48 PM
> >
> > 在 2025/5/23 16:10, Wei Wang 写道:
> > > This original implementation included redundant logic to determine
> > whether
> > > first-stage translation should be used by default. Simplify it and
> > > preserve the original behavior:
> > > - Returns false in legacy mode (no scalable mode support).
> > > - Defaults to first-level translation when both FLTS and SLTS are
> > >     supported.
> > >
> > > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> > > ---
> > >    drivers/iommu/intel/iommu.c | 10 +---------
> > >    1 file changed, 1 insertion(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > > index cb0b993bebb4..228da47ab7cd 100644
> > > --- a/drivers/iommu/intel/iommu.c
> > > +++ b/drivers/iommu/intel/iommu.c
> > > @@ -1366,15 +1366,7 @@ static void free_dmar_iommu(struct
> > intel_iommu *iommu)
> > >     */
> > >    static bool first_level_by_default(struct intel_iommu *iommu)
> > >    {
> > > -	/* Only SL is available in legacy mode */
> > > -	if (!sm_supported(iommu))
> > > -		return false;
> > > -
> > > -	/* Only level (either FL or SL) is available, just use it */
> > > -	if (ecap_flts(iommu->ecap) ^ ecap_slts(iommu->ecap))
> > > -		return ecap_flts(iommu->ecap);
> > > -
> > > -	return true;
> >
> > The function works like a digital circurt has 3 single bit inputs  sm,
> > flts, slts and one bit output ret.
> >
> > so the true value table of the orignal function looks like
> >
> >      sm   flts   slts    ret
> > a   0     x     x      false
> > b   1     1     0      true
> > c   1     0     1      false
> > d   1     1     1      true
> > e   1     0     0      true
>
> 'e' is actually wrong. We should not return true when the 1st level
> cap doesn't exist.

If so, this patch should mention it fixes such case with fix tag, not
 "removing the redundant logic with the same behavior".


Thanks,
Ethan
> > > +	return sm_supported(iommu) && ecap_flts(iommu->ecap);
> >
> > And the true value table of this new one looks like
> >
> >      sm  flts slts    ret
> >
> > f   1     1     x      true
> > g   1     0     x      false
> >
> > h   0     1     x      false
> > i    0     0     x      false
>
> so this table is correct.