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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v3 0/4] x86: accommodate 32-bit PV guests with SMEP/SMAP handling
>>> On 21.06.16 at 08:19, <feng.wu@xxxxxxxxx> wrote: >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx] >> Sent: Thursday, March 17, 2016 3:51 PM >> >> As has been explained previously[1], SMAP (and with less relevance >> also SMEP) is not compatible with 32-bit PV guests which aren't >> aware/prepared to be run with that feature enabled. Andrew's >> original approach either sacrificed architectural correctness for >> making 32-bit guests work again, or disabled SMAP also for not >> insignificant portions of hypervisor code, both by allowing to control >> the workaround mode via command line option. > > Hi Jan, do you mind sharing more about Andrew's original approach? > And to solve this issue can we just disable SMEP/SMAP for Xen itself, > hence only HVM will benefit from this feature? Did you look at the link still visible below? If you did, please be more precise about the details you need. >> This alternative approach disables SMEP and SMAP only while >> running 32-bit PV guest code plus a few hypervisor instructions >> early after entering hypervisor context from or late before exiting >> to such guests. Those few instructions (in assembly source) are of >> course much easier to prove not to perform undue memory >> accesses than code paths reaching deep into C sources. >> >> The 4th patch really is unrelated except for not applying cleanly >> without the earlier ones, and the potential having been noticed >> while putting together the 2nd one. >> >> 1: move cached CR4 value to struct cpu_info >> 2: suppress SMEP and SMAP while running 32-bit PV guest code >> 3: use optimal NOPs to fill the SMEP/SMAP placeholders >> 4: use 32-bit loads for 32-bit PV guest state reload >> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> >> --- >> v3: New patch 1, as a prereq to changes in patch 2 (previously >> 1). The primary reason for this are performance issues that >> have been found by Andrew with the previous version. >> v2: Various changes to patches 1 and 2 - see there. >> >> [1] >> http://lists.xenproject.org/archives/html/xen-devel/2015-06/msg03888.html _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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