On Wed, Sep 14, 2022 at 9:05 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Sep 08, 2022, Michael Roth wrote: > > On Fri, Oct 15, 2021 at 05:16:28PM +0000, Sean Christopherson wrote: > > So in the context of this interim solution, we're trying to look for a > > solution that's simple enough that it can be used reliably, without > > introducing too much additional complexity into KVM. There is one > > approach that seems to fit that bill, that Brijesh attempted in an > > earlier version of this series (I'm not sure what exactly was the > > catalyst to changing the approach, as I wasn't really in the loop at > > the time, but AIUI there weren't any showstoppers there, but please > > correct me if I'm missing anything): > > > > - if the host is writing to a page that it thinks is supposed to be > > shared, and the guest switches it to private, we get an RMP fault > > (actually, we will get a !PRESENT fault, since as of v5 we now > > remove the mapping from the directmap as part of conversion) > > - in the host #PF handler, if we see that the page is marked private > > in the RMP table, simply switch it back to shared > > - if this was a bug on the part of the host, then the guest will see > > As discussed off-list, attempting to fix up RMP violations in the host #PF handler > is not a viable approach. There was also extensive discussion on-list a while back: > > https://lore.kernel.org/all/8a244d34-2b10-4cf8-894a-1bf12b59cf92@xxxxxxxxxxxxxxxx I mentioned this during Mike's talk at the micro-conference: For pages mapped in by the kernel can we disallow them to be converted to private? Note, userspace accesses are already handled by UPM. In pseudo-code, I'm thinking something like this: kmap_helper() { // And all other interfaces where the kernel can map a GPA // into the kernel page tables mapped_into_kernel_mem_set[hpa] = true; } kunmap_helper() { // And all other interfaces where the kernel can unmap a GPA // into the kernel page tables mapped_into_kernel_mem_set[hpa] = false; // Except it's not this simple because we probably need ref counting // for multiple mappings. Sigh. But you get the idea. } rmpupdate_helper() { if (conversion = SHARED_TO_PRIVATE && mapped_into_kernel_mem_set[hpa]) return -EINVAL; // Or whatever the appropriate error code here is. }
