On Fri, Jan 08, 2010 at 02:18:28PM -0200, Marcelo Tosatti wrote: > On Tue, Jan 05, 2010 at 04:12:42PM +0200, Gleb Natapov wrote: > > KVM virtualizes guest memory by means of shadow pages or HW assistance > > like NPT/EPT. Not all memory used by a guest is mapped into the guest > > address space or even present in a host memory at any given time. > > When vcpu tries to access memory page that is not mapped into the guest > > address space KVM is notified about it. KVM maps the page into the guest > > address space and resumes vcpu execution. If the page is swapped out > > from host memory vcpu execution is suspended till the page is not swapped > > into the memory again. This is inefficient since vcpu can do other work > > (run other task or serve interrupts) while page gets swapped in. > > > > To overcome this inefficiency this patch series implements "asynchronous > > page fault" for paravirtualized KVM guests. If a page that vcpu is > > trying to access is swapped out KVM sends an async PF to the vcpu > > and continues vcpu execution. Requested page is swapped in by another > > thread in parallel. When vcpu gets async PF it puts faulted task to > > sleep until "wake up" interrupt is delivered. When the page is brought > > to the host memory KVM sends "wake up" interrupt and the guest's task > > resumes execution. > > Some high level comments: > > - cr2 used as token: better use the shared region? what if: > > async pf queued > guest triple faults without a vmexit > inject async-pf-done with token in cr2 > > Also, in such scenario, can't you potentially corrupt guest memory after > the triple fault by writing to the previously registered shared region > address? > After triple faults guest will reboot and this should clear all pending async pf injections. I'll check that this is indeed happens. > - The token can overflow relatively easy. Use u64? It not only can it frequently does, but since there can't be 2^20 outstanding page faults per vcpu simultaneously this doesn't cause any problem. > > - Does it really inject interrupts for non-pv guests while waiting > for swapin? Can't see that. Wish it was more geared towards fv. > No it does not yet. I only started to play with this to see how it can work. > - Please share some perf numbers. > OK. > - Limit the number of queued async pf's per guest ? > Make sense. > - Unify gfn_to_pfn / gfn_to_pfn_async code in the pf handlers (easier > to review). OK. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
