Re: [Xen-devel] [PATCH v2 1/2] x86/dom0: rename paging function

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

On Wed, Dec 12, 2018 at 02:53:26AM -0700, Jan Beulich wrote:
> >>> On 12.12.18 at 10:14, <roger.pau@xxxxxxxxxx> wrote:
> > On Tue, Dec 11, 2018 at 08:33:08AM -0700, Jan Beulich wrote:
> >> >>> On 11.12.18 at 16:19, <roger.pau@xxxxxxxxxx> wrote:
> >> > On Tue, Dec 11, 2018 at 08:08:51AM -0700, Jan Beulich wrote:
> >> >> >>> On 05.12.18 at 15:54, <roger.pau@xxxxxxxxxx> wrote:
> >> >> > To note it's calculating the approximate amount of memory required by
> >> >> > shadow paging.
> >> >> 
> >> >> I don't understand this logic, and ...
> >> >> 
> >> >> > @@ -325,7 +325,7 @@ unsigned long __init dom0_compute_nr_pages(
> >> >> >              break;
> >> >> >  
> >> >> >          /* Reserve memory for shadow or HAP. */
> >> >> > -        avail -= dom0_paging_pages(d, nr_pages);
> >> >> > +        avail -= dom0_shadow_pages(d, nr_pages);
> >> >> >      }
> >> >> 
> >> >> ... the comment here (and lack of conditional restricting the
> >> >> code to shadow mode) appear to support me: Have you
> >> >> been mislead by the function having a comment referring
> >> >> to libxl_get_required_shadow_memory()? I think if anything
> >> >> that libxl function would want to be renamed (to replace
> >> >> "shadow" by something more generic in its name).
> >> > 
> >> > But the logic in dom0_shadow_pages to calculate the size of the paging
> >> > memory pool is specifically for shadow AFAICT, I don't think HAP needs
> >> > to take the number of vCPUs into account, since there's only a
> >> > single p2m for the whole domain. OTOH shadow needs to take the number
> >> > of vCPUs into account because each one will have a different shadow.
> >> 
> >> Yes, the vCPU count aspect is indeed shadow specific. However,
> >> as said in reply to the other patch, the calculation here was at
> >> least supposed to also take into account the P2M part of the
> >> needed allocations. Yet the P2M part ought to be similar between
> >> both modes.
> >> 
> >> > Note that patch 2 in this series adds a function to calculate the size
> >> > of the paging memory pool for HAP, and a conditional is added to the
> >> > expression above that takes into account whether shadow or HAP is in
> >> > use when subtracting from the amount of available memory.
> >> 
> >> Well, assuming we can settle on what shape patch 2 should take
> >> I can see the point in doing the rename here, but then with an
> >> adjusted description: Especially in light of the code comment still
> >> visible above you'll want to point out that the rename is in
> >> preparation of splitting the calculations. Since I question the split,
> >> though, the rename (in a separate patch) is questionable to me
> >> too. If we used uniform P2M calculations and added just shadow's
> >> per-vCPU extra on top, no rename in a separate patch would
> >> seem warranted.
> > 
> > The current calculations in dom0_paging_pages assume 1 page is needed
> > for each 1MB of guest memory for the p2m, do you think this is OK?
> > (and suitable to be used for HAP/IOMMU page tables also)
> 
> Well, 1 page per 1Mb means the same as your current 8 bytes
> per page times 2 (for separate P2M and IOMMU tables), afaict.

I was planning to use 1 page per 1Mb for the p2m, and then 1 page per
1Mb for the IOMMU, so 16 bytes per page.

You mentioned there's some code (for PV?) to calculate the size of the
page tables but I'm having trouble finding it (mainly because I'm not
that familiar with PV), could you point me to it?

Thanks, Roger.

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