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

[Greg Kroah-Hartman]

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.   Please resubmit it after -rc1 is out.

thanks,

greg k-h