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xen-devel
[Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation
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To: |
Andi Kleen <andi@xxxxxxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation |
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From: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
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Date: |
Wed, 20 May 2009 09:39:45 -0700 |
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Cc: |
Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx> |
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Wed, 20 May 2009 09:40:22 -0700 |
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Andi Kleen wrote:
Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:
arch/x86 already defines an mtrr_ops, which defines how to manipulate
the MTRR registers. There are currently several implementations of
that interface. In Xen the MTRR registers belong to the hypervisor,
but it allows a privileged kernel to modify them via hypercalls.
One part that's unclear to me in this discussion. Could you perhaps
clarify Jeremy?:
Even Dom0 is not continuous in physical memory, but mapped page by page
except for swiotlb mappings. But MTRRs are fundamentally a way
to change attributes for large physically continous mappings. How do these
two meet?
After all when you change a MTRR for a given range of memory
linux sees as continuous it isn't necessarily in Xen.
Is this new interface only defined for swiotlb or MMIO mappings?
If yes did you check the drivers only actually set it on
swiotlb or MMIO (that seems dubious to me)?
Really? Do we ever set unusual memory types on normal system memory?
From what I've seen, MTRRs are only ever applied to device mapped
memory (framebuffers, etc). I guess it could possibly make sense on
system memory which is being prepped for DMA (swiotlb, alloc_coherent,
etc), but dom0 would have a pseudo-phys to machine mapping for that
memory too (it would be obviously problematic if something tried to
program MTRR with pseudo-physical addresses, but Xen would/should
probably disallow it anyway).
J
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