[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] ARM: cache coherence problem in guestcopy.c
On Thu, 2013-06-20 at 13:19 +0100, Tim Deegan wrote: > At 12:55 +0100 on 20 Jun (1371732938), Stefano Stabellini wrote: > > On Tue, 18 Jun 2013, Ian Campbell wrote: > > > On Tue, 2013-06-18 at 12:05 +0000, Jaeyong Yoo wrote: > > > > > We were also speculating that we probably want some DMBs in > > > > > context_switch_{from,to} as well as at return_to_guest. > > > > > > > > Actually, I just learned ReOrder Buffer, and it looks like so. > > > > > > Right. Tim and I were speculating about where and why the barriers were > > > needed: > > > > > > In unmap_domain_page: This ensures any writes to domain memory via the > > > Xen mapping are seen by subsequent accesses via the guest mapping. > > > Likewise map_domain_page needs one to ensure that previous accesses via > > > the guest mapping will be observed (this is the issue you observed). > > > > What if the guest doesn't have caching enabled? > > Erk! I think we can mandate that the guest use caching for memory it > shares with the hypervisor (i.e. hypercall args & responses, shared > pages, granted pages). Otherwise we end up flushing the dcache before > and after every copy. Yes. Worse because of the reusing/reference counting scheme used in map_domain_page I'm not sure we can tell when we need to flush the cache vs. discard it (because the guest has written something useful behind the cache). > In any case, this is something that should get documented. :) Yes. > > > In context_switch_from: To ensure any writes done on a VCPU are seen > > > before it is rescheduled, potentially on another PCPU. It seems likely > > > that you'd want one in context_switch_to for the complementary reasons, > > > > Is the cache shared across multiple pcpus? Because if it isn't then we > > need to flush the dcache in this case. > > AIUI this is what marking accesses 'shareable' is for: to tell the caches > to keep them coherent. If not, we're equally in trouble for all Xen's > internal data structures. Right they are either snooped or shared, depending on the level and the implementation. Either way it is fine for us I think. Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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