Re: [PATCH v2 0/7] Allow dynamic allocation of software IO TLB bounce buffers

[Petr Tesařík]

Hi Greg,

On Wed, 26 Apr 2023 14:26:36 +0200
Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, Apr 26, 2023 at 02:15:20PM +0200, Petr Tesařík wrote:
> > Hi,
> > 
> > On Wed, 19 Apr 2023 12:03:52 +0200
> > Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> wrote:
> >   
> > > From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
> > > 
> > > The goal of my work is to provide more flexibility in the sizing of
> > > SWIOTLB.
> > > 
> > > The software IO TLB was designed with these assumptions:
> > > 
> > > 1. It would not be used much, especially on 64-bit systems.
> > > 2. A small fixed memory area (64 MiB by default) is sufficient to
> > >    handle the few cases which require a bounce buffer.
> > > 3. 64 MiB is little enough that it has no impact on the rest of the
> > >    system.
> > > 
> > > First, if SEV is active, all DMA must be done through shared
> > > unencrypted pages, and SWIOTLB is used to make this happen without
> > > changing device drivers. The software IO TLB size is increased to
> > > 6% of total memory in sev_setup_arch(), but that is more of an
> > > approximation. The actual requirements may vary depending on the
> > > amount of I/O and which drivers are used. These factors may not be
> > > know at boot time, i.e. when SWIOTLB is allocated.
> > > 
> > > Second, other colleagues have noticed that they can reliably get
> > > rid of occasional OOM kills on an Arm embedded device by reducing
> > > the SWIOTLB size. This can be achieved with a kernel parameter, but
> > > determining the right value puts additional burden on pre-release
> > > testing, which could be avoided if SWIOTLB is allocated small and
> > > grows only when necessary.  
> > 
> > Now that merging into 6.4 has begun, what about this patch series? I'm
> > eager to get some feedback (positive or negative) and respin the next
> > version.  
> 
> It's the merge window, we can't add new things that haven't been in
> linux-next already.

This is understood. I'm not asking for immediate inclusion.

>   Please resubmit it after -rc1 is out.

If you can believe that rebasing to -rc1 will be enough, then I will
also try to believe I'm lucky. ;-)

The kind of feedback I really want to get is e.g. about the extra
per-device DMA-specific fields. If they cannot be added to struct
device, then I'd rather start discussing an interim solution, because
getting all existing DMA fields out of that struct will take a lot of
time...

Thanks,
Petr T