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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <DS0PR11MB637325E3CD8FECC9A1D1213ADC6FA@DS0PR11MB6373.namprd11.prod.outlook.com>
Date: Thu, 5 Jun 2025 01:04:28 +0000
From: "Wang, Wei W" <wei.w.wang@...el.com>
To: Ethan Zhao <etzhao1900@...il.com>, "Tian, Kevin" <kevin.tian@...el.com>,
	Ethan Zhao <haifeng.zhao@...ux.intel.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()

On Wednesday, June 4, 2025 3:26 PM, Ethan Zhao wrote:
> On 5/29/2025 2:11 PM, Tian, Kevin wrote:
> >> 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.
> 
> If so, this patch should mention it fixes such case with fix tag, not
>   "removing the redundant logic with the same behavior".
> 

Probably not a fix (just code improvement). Your 'e' above is not possible
from the hardware side, and the old implementation doesn't have it too.


> >
> >>
> >>> +	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.