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Re: [Xen-devel] [PATCH 5/5] x86/dom0: take alignment into account when populating p2m in PVH mode
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 27 Dec 2018 20:14:40 +0000
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Thu, 27 Dec 2018 20:15:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
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On 27/12/2018 15:26, Roger Pau Monne
wrote:
Current code that allocates memory and populates the p2m for PVH Dom0
doesn't take the address alignment into account, this can lead to high
order allocations that start on a non-aligned address to be broken
down into lower order entries on the p2m page tables.
Fix this by taking into account the p2m page sizes and alignment
requirements when allocating the memory and populating the p2m.
Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
I've committed patches 1-4 because they are trivial.
This patch does fix the alignment issue, but does so at a rather
large cost.
The sequence (without any of the E820 series) is now:
(XEN) *** Building a PVH Dom0 ***
(XEN) pvh_populate_memory_range(d0, 0, a0)
(XEN) guest_physmap_add_page(d0, 00000, 100be1, 0 = 4kB)
...
(XEN) guest_physmap_add_page(d0, 0009f, 100b42, 0 = 4kB)
(XEN) pvh_populate_memory_range(d0, 100, 360)
(XEN) guest_physmap_add_page(d0, 00100, 100b41, 0 = 4kB)
...
(XEN) guest_physmap_add_page(d0, 001ff, 100a42, 0 = 4kB)
(XEN) guest_physmap_add_page(d0, 00200, 100800, 9 = 2048kB)
(XEN) guest_physmap_add_page(d0, 00400, 100a41, 0 = 4kB)
...
(XEN) guest_physmap_add_page(d0, 0045f, 1007e2, 0 = 4kB)
So overall, the 2M superpage is created in the middle, but all the
4k entries are made with a single call to guest_physmap_add_page()
at a time, which is going to be very inefficient with the P2M lock.
Ideally, we should make a single order 8 allocation at 1M, then the
order 9 at 2M.
I think it might be better to try and crib from the HVM
domainbuilder code, which copes with collecting together the head
and tail of non-aligned allocations while still making an order >
0 requests where possible.
~Andrew
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