linux-kernel - RE: [PATCH v1] iommu/vt-d: Remove the redundant logic in first_level_by

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Message-ID: <BN9PR11MB5276CFBD4CF795CCB3ADCD1B8C66A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 29 May 2025 06:11:00 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Ethan Zhao <haifeng.zhao@...ux.intel.com>, "Wang, Wei W"
	<wei.w.wang@...el.com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>, "jroedel@...e.de"
	<jroedel@...e.de>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>
Subject: RE: [PATCH v1] iommu/vt-d: Remove the redundant logic in
 first_level_by_default()

> From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
> 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@...el.com>
> > ---
> >   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.

> 
> > +	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.