Re: [PATCH v2 08/18] IOMMU/x86: support freeing of pagetables

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Dec 02, 2021 at 05:10:38PM +0100, Jan Beulich wrote:
> On 02.12.2021 17:03, Roger Pau Monné wrote:
> > On Fri, Sep 24, 2021 at 11:48:21AM +0200, Jan Beulich wrote:
> >> For vendor specific code to support superpages we need to be able to
> >> deal with a superpage mapping replacing an intermediate page table (or
> >> hierarchy thereof). Consequently an iommu_alloc_pgtable() counterpart is
> >> needed to free individual page tables while a domain is still alive.
> >> Since the freeing needs to be deferred until after a suitable IOTLB
> >> flush was performed, released page tables get queued for processing by a
> >> tasklet.
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> ---
> >> I was considering whether to use a softirq-taklet instead. This would
> >> have the benefit of avoiding extra scheduling operations, but come with
> >> the risk of the freeing happening prematurely because of a
> >> process_pending_softirqs() somewhere.
> > 
> > Another approach that comes to mind (maybe you already thought of it
> > and discarded) would be to perform the freeing after the flush in
> > iommu_iotlb_flush{_all} while keeping the per pPCU lists.
> 
> This was my initial plan, but I couldn't convince myself that the first
> flush to happen would be _the_ one associated with the to-be-freed
> page tables. ISTR (vaguely though) actually having found an example to
> the contrary.

I see. If we keep the list per pCPU I'm not sure we could have an
IOMMU  flush not related to the to-be-freed pages, as we cannot have
interleaved IOMMU operations on the same pCPU.

Also, if we strictly add the pages to the freeing list once unhooked
from the IOMMU page tables it should be safe to flush and then free
them, as they would be no references remaining anymore.

Thanks, Roger.