On Wed, Dec 4, 2024, at 19:37, Andy Shevchenko wrote:
> On Wed, Dec 4, 2024 at 6:57 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Wed, Dec 4, 2024, at 17:37, H. Peter Anvin wrote:
>> > On December 4, 2024 5:29:17 AM PST, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> >>>
>> >>> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOTLB.
>> >>> PAE mode is still required to get access to the 'NX' bit on Atom
>> >>> 'Pentium M' and 'Core Duo' CPUs.
>> >
>> > By the way, there are 64-bit machines which require swiotlb.
>>
>> What I meant to write here was that CONFIG_X86_PAE no longer
>> needs to select PHYS_ADDR_T_64BIT and SWIOTLB. I ended up
>> splitting that change out to patch 06/11 with a better explanation,
>> so the sentence above is just wrong now and I've removed it
>> in my local copy now.
>>
>> Obviously 64-bit kernels still generally need swiotlb.
>
> Theoretically swiotlb can be useful on 32-bit machines as well for the
> DMA controllers that have < 32-bit mask. Dunno if swiotlb was designed
> to run on 32-bit machines at all.
Right, that is a possibility. However those machines would
currently be broken on kernels without X86_PAE, since they
don't select swiotlb.
If anyone does rely on the current behavior of X86_PAE to support
broken DMA devices, it's probably best to fix it in a different
way.
Arnd
