On Wed, Jan 15, 2020 at 01:50:08AM -0500, Michael S. Tsirkin wrote: > On Tue, Jan 14, 2020 at 03:01:34PM -0500, Peter Xu wrote: > > On Thu, Jan 09, 2020 at 02:35:46PM -0500, Michael S. Tsirkin wrote: > > > ``void flush_dcache_page(struct page *page)`` > > > > > > Any time the kernel writes to a page cache page, _OR_ > > > the kernel is about to read from a page cache page and > > > user space shared/writable mappings of this page potentially > > > exist, this routine is called. [1] > > > > > > > > > > Also, I believe this is the similar question that Jason has asked in > > > > V2. Sorry I should mention this earlier, but I didn't address that in > > > > this series because if we need to do so we probably need to do it > > > > kvm-wise, rather than only in this series. > > > > > > You need to document these things. > > > > > > > I feel like it's missing > > > > probably only because all existing KVM supported archs do not have > > > > virtual-tagged caches as you mentioned. > > > > > > But is that a fact? ARM has such a variety of CPUs, > > > I can't really tell. Did you research this to make sure? > > > > > > > If so, I would prefer if you > > > > can allow me to ignore that issue until KVM starts to support such an > > > > arch. > > > > > > Document limitations pls. Don't ignore them. > > > > Hi, Michael, > > > > I failed to find a good place to document about flush_dcache_page() > > for KVM. Could you give me a suggestion? > > Maybe where the field is introduced. I posted the suggestions to the > relevant patch. (will reply there) > > > And I don't know about whether there's any ARM hosts that requires > > flush_dcache_page(). I think not, because again I didn't see any > > caller of flush_dcache_page() in KVM code yet. Otherwise I think we > > should at least call it before the kernel reading kvm_run or after > > publishing data to kvm_run. > > But is kvm run ever accessed while VCPU is running on another CPU? > I always assumed no but maybe I'm missing something? IMHO we need to call it even if it's running on the same CPU - please refer to [1] above, there's no restriction on which CPU the code is running on. I think it makes sense, especially the systems for virtually-tagged caches because even if the memory accesses happened on the same CPU, the virtual addresses to access the same page could still be different when accessed from kernel/userspace. Thanks, -- Peter Xu
