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Re: [Xen-devel] [PATCH] x86: Get rid of p2m_host array allocation for HVM guests
- To: Varad Gautam <vrd@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 3 Jul 2019 20:39:48 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Amit Shah <aams@xxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>, David Woodhouse <dwmw@xxxxxxxxxxxx>
- Delivery-date: Wed, 03 Jul 2019 19:39:58 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 03/07/2019 14:06, Varad Gautam wrote:
> When allocating the guest memory for an HVM domain, libxc keeps the P2M
> mapping for the entirety of the guest memory around for the time of the
> launch as xc_dom_image->p2m_host. For guests that have a large memory
> (3904 GiB),
This is slightly awkward phrasing. How about "For large memory guests
(e.g. 3904 GiB), "
> the p2m_host allocation takes more than 7.5 GiB of space, and
> leaves xl susceptible to getting OOM-killed on guest creation.
>
> Convert the p2m_host table lookups to an arch-specific function that
> returns the mapping on-the-fly for x86 HVM guests to avoid this
> allocation, bringing down xl's memory usage from > 8GiB to < 70Mib for
> such launches.
>
> Signed-off-by: Varad Gautam <vrd@xxxxxxxxx>
I can definitely see why you want to avoid this overhead.
In practice, p2m_host looks to be a relic of x86 PV guests.
Now - first things first. Exactly the same reasoning applies to all ARM
guests. (CC'ing the ARM maintainers for visibility)
However, in attempting to review this, I've got some bigger questions.
All ARM and x86 HVM (and PVH) guests return true for
xc_dom_translated(), so should take the fastpath out of xc_dom_p2m() and
never read from dom->p2m_host[]. Therefore, I don't see why the
majority of this patch is necessary. On the ARM side, this also means
that dom->rambase_pfn isn't being used as intended, which suggests there
is further cleanup/correction to be done here.
xc_dom_update_guest_p2m() is clearly specific to x86 PV guests. No
other guest types set dom->p2m_guest.
I don't see why anything is needed with the vmemranges[]. The result of
the new p2m_host() hook never has its return value checked, and
dom->p2m_host is still an identity transform within those ranges.
Unless I'm missing something, I think the result can be rather more
simple, and strip out a fair amount of code, by moving
p2m_host/p2m_guest into x86 PV's arch_private area.
~Andrew
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