> On Apr 19, 2021, at 11:33 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > On 4/19/21 11:10 AM, Andy Lutomirski wrote: >> I’m confused by this scenario. This should only affect physical pages >> that are in the 2M area that contains guest memory. But, if we have a >> 2M direct map PMD entry that contains kernel data and guest private >> memory, we’re already in a situation in which the kernel touching >> that memory would machine check, right? > > Not machine check, but page fault. Do machine checks even play a > special role in SEV-SNP? I thought that was only TDX? Brain fart. > > My point was just that you can't _easily_ do the 2M->4k kernel mapping > demotion in a page fault handler, like I think Borislav was suggesting. We are certainly toast if this hits the stack. Or if it hits a page table or the GDT or IDT :). The latter delightful choices would be triple faults. I sure hope the code we use to split a mapping is properly NMI safe. > >> ISTM we should fully unmap any guest private page from the kernel and >> all host user pagetables before actually making it be a guest private >> page. > > Yes, that sounds attractive. Then, we'd actually know if the host > kernel was doing stray reads somehow because we'd get a fault there too.
