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xen-devel
[Xen-devel] Re: [PATCH] xen: core dom0 support
Nick Piggin wrote:
On Monday 02 March 2009 10:27:29 Jeremy Fitzhardinge wrote:
Once important area of paravirtualization is that Xen guests directly
use the processor's pagetables; there is no shadow pagetable or use of
hardware pagetable nesting. This means that a tlb miss is just a tlb
miss, and happens at full processor performance. This is possible
because 1) pagetables are always read-only to the guest, and 2) the
guest is responsible for looking up in a table to map guest-local pfns
into machine-wide mfns before installing them in a pte. Xen will check
that any new mapping or pagetable satisfies all the rules, by checking
that the writable reference count is 0, and that the domain owns (or has
been allowed access to) any mfn it tries to install in a pagetable.
Xen's memory virtualization is pretty neat, I'll give it that. Is it
faster than KVM on a modern CPU?
There is nothing architecturally that prevents KVM from making use of
Direct Paging. KVM doesn't use Direct Paging because we don't expect it
will not be worth it. Modern CPUs (Barcelona and Nehalem class) include
hardware support for MMU virtualization (via NPT and EPT respectively).
I think that for the most part (especially with large page backed
guests), there's wide agreement that even within the context of Xen,
NPT/EPT often beats PV performance. TLB miss overhead increases due to
additional memory accesses but this is largely mitigated by large pages
(see Ben Serebin's SOSP paper from a couple years ago).
Would it be possible I wonder to make
a MMU virtualization layer for CPUs without support, using Xen's page
table protection methods, and have KVM use that? Or does that amount
to putting a significant amount of Xen hypervisor into the kernel..?
There are various benchmarks out there (check KVM Forum and Xen Summit
presentations) showing NPT/EPT beating Direct Paging but FWIW the direct
paging could be implemented in KVM.
A really unfortunate aspect of direct paging is that it requires the
guest to know the host physical addresses. This requires the guest to
cooperate when doing any fancy memory tricks (live migration,
save/restore, swapping, page sharing, etc.). This introduces guest code
paths to ensure that things like live migration works which is extremely
undesirable.
FWIW, I'm not advocating not taking the Xen dom0 patches. Just pointing
out that direct paging is orthogonal to the architectural differences
between Xen and KVM.
Regards,
Anthony Liguori
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