Re: [PATCH] memory: arrange to conserve on DMA reservation

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Jun 16, 2025 at 06:02:07PM +0200, Jan Beulich wrote:
> On 16.06.2025 17:41, Roger Pau Monné wrote:
> > On Mon, Jun 16, 2025 at 05:20:45PM +0200, Jan Beulich wrote:
> >> On 16.06.2025 16:46, Roger Pau Monné wrote:
> >>> One question I have though, on systems with a low amount of memory
> >>> (let's say 8GB), does this lead to an increase in domain construction
> >>> time due to having to fallback to order 0 allocations when running out
> >>> of non-DMA memory?
> >>
> >> It'll likely be slower, yes, but I can't guesstimate by how much.
> > 
> > Should there be some way to control this behavior then?  I'm mostly
> > thinking about client systems like Qubes where memory is likely
> > limited, and the extra slowness to create VMs could become
> > noticeable?
> 
> What kind of control would you be thinking of here? Yet another command
> line option?

I guess that would be enough.  I think we need a way to resort to the
previous behavior if required, and likely a CHANGELOG entry to notice
the change.

Overall, would it be possible to only include the flag if we know
there's non-DMA memory available to allocate?  Otherwise we are
crippling allocation performance when there's only DMA memory left.

That also raises the question whether it's an acceptable trade-off to
possibly shatter p2m super pages (that could be used if allocating
from the DMA pool) at the expense of not allocating from the DMA pool
until there's non-DMA memory left.

Thanks, Roger.