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

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

[Wei Wang]

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@intel.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;
+	return sm_supported(iommu) && ecap_flts(iommu->ecap);
 }
 
 int domain_attach_iommu(struct dmar_domain *domain, struct intel_iommu *iommu)
-- 
2.43.0

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

[Tian, Kevin]

> From: Wang, Wei W <wei.w.wang@intel.com>
> Sent: Friday, May 23, 2025 4:11 PM
> 
> 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@intel.com>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>

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

[Ethan Zhao]

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@intel.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

> +	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

h, i covers a
g,   doesn't cover e, c
f,   covers b, d

So if we take them as two circurt blocks (chip), their functions aren't 
equal to each other.

If we don't make any assumption,  I wouldn't replace the old chip with 
new one

But, maybe all the new chip and old one all works well under specific 
context that I

didn't think deeper. :)



Thanks,

Ethan

>   }
>   
>   int domain_attach_iommu(struct dmar_domain *domain, struct intel_iommu *iommu)

-- 
"firm, enduring, strong, and long-lived"

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

[Tian, Kevin]

> From: Ethan Zhao <haifeng.zhao@linux.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@intel.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.

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@linux.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@intel.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".


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.

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

[Wang, Wei W]

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@linux.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@intel.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.

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

[Ethan Zhao]

On 6/5/2025 9:04 AM, Wang, Wei W wrote:
> 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@linux.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@intel.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.
> 
Is there any items in Intel VT-d spec to say "It is impossible to have 
both flts and slts to false" or you design code under an assumption that 
hardware always works perfect ?

You made an assumption that the hardware never would reach there and 
then got a conclusion 'e' doesn't exist ?


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.
>