On Wed, Sep 14, 2022, Marc Orr wrote: > 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? In theory, yes. Do we want to do something like this? No. kmap() does something vaguely similar for 32-bit PAE/PSE kernels, but that's a lot of complexity and overhead to take on. And this issue goes far beyond a kmap(); when the kernel gup()s a page, the kernel expects the pfn to be available, no exceptions (pun intended). > Note, userspace accesses are already handled by UPM. I'm confused by the UPM comment. Isn't the gist of this thread about the ability to merge SNP _without_ UPM? Or am I out in left field? > 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. A few issues off the top of my head: - It's not just refcounting, there would also likely need to be locking to guarantee sane behavior. - kmap() isn't allowed to fail and RMPUPDATE isn't strictly guaranteed to succeed, which is problematic if the kernel attempts to kmap() a page that's already private, especially for kmap_atomic(), which isn't allowed to sleep. - Not all kernel code is well behaved and bounces through kmap(); undoubtedly some of the 1200+ users of page_address() will be problematic. $ git grep page_address | wc -l 1267 - It's not sufficient for TDX. Merging something this complicated when we know we still need UPM would be irresponsible from a maintenance perspective. - KVM would need to support two separate APIs for SNP, which I very much don't want to do.
