Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap

Marc Orr
2022-09-14T11:02:36Z

On Wed, Sep 14, 2022 at 9:05 AM Sean Christopherson <seanjc@google.com> 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@www.fastmail.com

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.
}