Message-ID: <CAMkAt6pcsgp7BK4WGnvTTNayN9zD8wx5CjprnY2Xe_RnpP3sEA@mail.gmail.com>
Date:   Fri, 24 Jun 2022 08:19:52 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
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Subject: Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR
 allocation when SNP is enabled

On Tue, Jun 21, 2022 at 2:17 PM Kalra, Ashish <Ashish.Kalra@....com> wrote:
>
> [Public]
>
> Hello Peter,
>
> >> +static int snp_reclaim_pages(unsigned long pfn, unsigned int npages,
> >> +bool locked) {
> >> +       struct sev_data_snp_page_reclaim data;
> >> +       int ret, err, i, n = 0;
> >> +
> >> +       for (i = 0; i < npages; i++) {
>
> >What about setting |n| here too, also the other increments.
>
> >for (i = 0, n = 0; i < npages; i++, n++, pfn++)
>
> Yes that is simpler.
>
> >> +               memset(&data, 0, sizeof(data));
> >> +               data.paddr = pfn << PAGE_SHIFT;
> >> +
> >> +               if (locked)
> >> +                       ret = __sev_do_cmd_locked(SEV_CMD_SNP_PAGE_RECLAIM, &data, &err);
> >> +               else
> >> +                       ret = sev_do_cmd(SEV_CMD_SNP_PAGE_RECLAIM,
> >> + &data, &err);
>
> > Can we change `sev_cmd_mutex` to some sort of nesting lock type? That could clean up this if (locked) code.
>
> > +static inline int rmp_make_firmware(unsigned long pfn, int level) {
> > +       return rmp_make_private(pfn, 0, level, 0, true); }
> > +
> > +static int snp_set_rmp_state(unsigned long paddr, unsigned int npages, bool to_fw, bool locked,
> > +                            bool need_reclaim)
>
> >This function can do a lot and when I read the call sites its hard to see what its doing since we have a combination of arguments which tell us what behavior is happening, some of which are not valid (ex: to_fw == true and need_reclaim == true is an >invalid argument combination).
>
> to_fw is used to make a firmware page and need_reclaim is for freeing the firmware page, so they are going to be mutually exclusive.
>
> I actually can connect with it quite logically with the callers :
> snp_alloc_firmware_pages will call with to_fw = true and need_reclaim = false
> and snp_free_firmware_pages will do the opposite, to_fw = false and need_reclaim = true.
>
> That seems straightforward to look at.

This might be a preference thing but I find it not straightforward.
When I am reading through unmap_firmware_writeable() and I see

  /* Transition the pre-allocated buffer to the firmware state. */
  if (snp_set_rmp_state(__pa(map->host), npages, true, true, false))
   return -EFAULT;

I don't actually know what snp_set_rmp_state() is doing unless I go
look at the definition and see what all those booleans mean. This is
unlike the rmp_make_shared() and rmp_make_private() functions, each of
which tells me a lot more about what the function will do just from
the name.


>
> >Also this for loop over |npages| is duplicated from snp_reclaim_pages(). One improvement here is that on the current
> >snp_reclaim_pages() if we fail to reclaim a page we assume we cannot reclaim the next pages, this may cause us to snp_leak_pages() more pages than we actually need too.
>
> Yes that is true.
>
> >What about something like this?
>
> >static snp_leak_page(u64 pfn, enum pg_level level) {
> >   memory_failure(pfn, 0);
> >   dump_rmpentry(pfn);
> >}
>
> >static int snp_reclaim_page(u64 pfn, enum pg_level level) {
> >  int ret;
> >  struct sev_data_snp_page_reclaim data;
>
> >  ret = sev_do_cmd(SEV_CMD_SNP_PAGE_RECLAIM, &data, &err);
> >  if (ret)
> >    goto cleanup;
>
> >  ret = rmp_make_shared(pfn, level);
> >  if (ret)
> >    goto cleanup;
>
> > return 0;
>
> >cleanup:
> >    snp_leak_page(pfn, level)
> >}
>
> >typedef int (*rmp_state_change_func) (u64 pfn, enum pg_level level);
>
> >static int snp_set_rmp_state(unsigned long paddr, unsigned int npages, rmp_state_change_func state_change, rmp_state_change_func cleanup) {
> >  struct sev_data_snp_page_reclaim data;
> >  int ret, err, i, n = 0;
>
> >  for (i = 0, n = 0; i < npages; i++, n++, pfn++) {
> >    ret = state_change(pfn, PG_LEVEL_4K)
> >    if (ret)
> >      goto cleanup;
> >  }
>
> >  return 0;
>
> > cleanup:
> >  for (; i>= 0; i--, n--, pfn--) {
> >    cleanup(pfn, PG_LEVEL_4K);
> >  }
>
> >  return ret;
> >}
>
> >Then inside of __snp_alloc_firmware_pages():
>
> >snp_set_rmp_state(paddr, npages, rmp_make_firmware, snp_reclaim_page);
>
> >And inside of __snp_free_firmware_pages():
>
> >snp_set_rmp_state(paddr, npages, snp_reclaim_page, snp_leak_page);
>
> >Just a suggestion feel free to ignore. The readability comment could be addressed much less invasively by just making separate functions for each valid combination of arguments here. Like snp_set_rmp_fw_state(), snp_set_rmp_shared_state(),
> >snp_set_rmp_release_state() or something.
>
> >> +static struct page *__snp_alloc_firmware_pages(gfp_t gfp_mask, int
> >> +order, bool locked) {
> >> +       unsigned long npages = 1ul << order, paddr;
> >> +       struct sev_device *sev;
> >> +       struct page *page;
> >> +
> >> +       if (!psp_master || !psp_master->sev_data)
> >> +               return NULL;
> >> +
> >> +       page = alloc_pages(gfp_mask, order);
> >> +       if (!page)
> >> +               return NULL;
> >> +
> >> +       /* If SEV-SNP is initialized then add the page in RMP table. */
> >> +       sev = psp_master->sev_data;
> >> +       if (!sev->snp_inited)
> >> +               return page;
> >> +
> >> +       paddr = __pa((unsigned long)page_address(page));
> >> +       if (snp_set_rmp_state(paddr, npages, true, locked, false))
> >> +               return NULL;
>
> >So what about the case where snp_set_rmp_state() fails but we were able to reclaim all the pages? Should we be able to signal that to callers so that we could free |page| here? But given this is an error path already maybe we can optimize this in a >follow up series.
>
> Yes, we should actually tie in to snp_reclaim_pages() success or failure here in the case we were able to successfully unroll some or all of the firmware state change.
>
> > +
> > +       return page;
> > +}
> > +
> > +void *snp_alloc_firmware_page(gfp_t gfp_mask) {
> > +       struct page *page;
> > +
> > +       page = __snp_alloc_firmware_pages(gfp_mask, 0, false);
> > +
> > +       return page ? page_address(page) : NULL; }
> > +EXPORT_SYMBOL_GPL(snp_alloc_firmware_page);
> > +
> > +static void __snp_free_firmware_pages(struct page *page, int order,
> > +bool locked) {
> > +       unsigned long paddr, npages = 1ul << order;
> > +
> > +       if (!page)
> > +               return;
> > +
> > +       paddr = __pa((unsigned long)page_address(page));
> > +       if (snp_set_rmp_state(paddr, npages, false, locked, true))
> > +               return;
>
> > Here we may be able to free some of |page| depending how where inside of snp_set_rmp_state() we failed. But again given this is an error path already maybe we can optimize this in a follow up series.
>
> Yes, we probably should be able to free some of the page(s) depending on how many page(s) got reclaimed in snp_set_rmp_state().
> But these reclamation failures may not be very common, so any failure is indicative of a bigger issue, it might be the case when there is a single page reclamation error it might happen with all the subsequent
> pages and so follow a simple recovery procedure, then handling a more complex recovery for a chunk of pages being reclaimed and another chunk not.
>
> Thanks,
> Ashish
>
>
>

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