Re: [PATCH v1 03/11] x86/mm/pat: introduce pfnmap_track() and pfnmap_untrack()

Lorenzo Stoakes
2025-04-28T19:00:36Z

On Mon, Apr 28, 2025 at 07:12:11PM +0200, David Hildenbrand wrote:
>
> > >
> > > +int pfnmap_track(unsigned long pfn, unsigned long size, pgprot_t *prot)
> > > +{
> > > + const resource_size_t paddr = (resource_size_t)pfn << PAGE_SHIFT;
> > > +
> > > + return reserve_pfn_range(paddr, size, prot, 0);
> >
> > Nitty, but a pattern established by Liam which we've followed consistently
> > in VMA code is to prefix parameters that might be less than obvious,
> > especially boolean parameters, with a comment naming the parameter, e.g.:
> > > return reserve_pfn_range(paddr, size, prot, /*strict_prot=*/0);
>
> Not sure I like that. But as this parameter goes away patch #8, I'll leave
> it as is in this patch and not start a bigger discussion on better
> alternatives (don't use these stupid boolean variables ...) ;)
>
> [...]
>
> > > +
> > > +/**
> > > + * pfnmap_track - track a pfn range
> >
> > To risk sounding annoyingly pedantic and giving the kind of review that is
> > annoying, this really needs to be expanded, I think perhaps this
> > description is stating the obvious :)
> >
> > To me the confusing thing is that the 'generic' sounding pfnmap_track() is
> > actually PAT-specific, so surely the description should give a brief
> > overview of PAT here, saying it's applicable on x86-64 etc. etc.
> >
> > I'm not sure there's much use in keeping this generic when it clearly is
> > not at this point?
>
> Sorry, is your suggestion to document more PAT stuff or what exactly?
>
> As you know, I'm a busy man ... so instructions/suggestions please :)

Haha sure, I _think_ the model here is to have a brief summary then underneath a
more detailed explanation, so that could be:

This address range is requested to be 'tracked' by a hardware
implementation allowing fine-grained control of memory attributes at
page level granularity.

This allows for fine-grained control of memory cache behaviour. Tracking
memory this way is persisted across VMA split and merge.

Currently there is only one implementation for this - x86 Page Attribute
Table (PAT). See Documentation/arch/x86/pat.rst for more details.

>
> >
> > > + * @pfn: the start of the pfn range
> > > + * @size: the size of the pfn range
> >
> > In what units? Given it's a pfn range it's a bit ambiguous as to whether it
> > should be expressed in pages/bytes.
>
> Agreed. It's bytes. (not my favorite here, but good enough)

Ack, definitely need to spell it out here! Cheers :)

>
>
> --
> Cheers,
>
> David / dhildenb
>