[Xen-devel] [PATCH][HVM][SVM] Reintroduce ASIDs.

["Biemueller, Sebastian" <Sebastian.Biemueller@xxxxxxx>]

This patch reintroduces ASID management for AMD-V.  This work was presented on 
the last XenSummit [1].

Short overview:

ASIDs partition the physical TLB for SVM.  In the current implementation ASIDs 
are used to reduce the number of TLB flushes.  Each time the guest's virtual 
address space changes (e.g. due to an INVLPG, MOV-TO-{CR3, CR4} operation), 
instead of flushing the TLB, a new ASID is assigned.  This reduces the number 
of TLB flushes to at most 1/#ASIDs (currently 1/64).  The biggest advantage is 
that hot parts of the hypervisor's code and data remain in the TLB.


Sketch of the Implementation:

ASIDs are a CPU-local resource.  As preemption of ASIDs is not possible, ASIDs 
are assigned in a round-robin scheme.  To minimize the overhead of ASID 
invalidation, at the time of a TLB flush, ASIDs are tagged with a 64-bit 
generation.  Generation overflows are designed not to happen and thus not 
optimized. (They appear at most every 2^80 cycles).  Here ASIDs are simply 
disabled to retain correctness [2].


b/xen/arch/x86/hvm/svm/asid.c        |  181 ++++++++++++++++++++++++++++++++
b/xen/include/asm-x86/hvm/svm/asid.h |  162 +++++++++++++++++++++++++++++++
xen/arch/x86/cpu/amd.c               |    4 
xen/arch/x86/hvm/svm/Makefile        |    1 
xen/arch/x86/hvm/svm/svm.c           |   41 ++++++-
xen/arch/x86/hvm/svm/vmcb.c          |    8 -
xen/arch/x86/hvm/svm/x86_32/exits.S  |    7 +
xen/arch/x86/hvm/svm/x86_64/exits.S  |    7 +
xen/include/asm-x86/hvm/svm/vmcb.h   |    5 
9 files changed, 401 insertions(+), 15 deletions(-



Regards,

 Sebastian



References:

[1] Presentation given at the XenSummit Spring, 2007
http://www.xensource.com/files/xensummit_4/2007XenSummit-AMD-ASIDS-Biemueller.pdf

[2] The 2^80 cycles result from: 1. And VM-Entry/-Exit cycle takes at least 
1800 cycles (approximated by 2^10) 2. We have 64 ASIDs (2^6)  3. 2^64 
generations.


--
AMD Inc.
Operating System Research Center

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Attachment: svm_asid_against_cs15017.patch
Description: svm_asid_against_cs15017.patch

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