On Fri, Aug 27, 2021 at 10:47:42AM -0500, Brijesh Singh wrote: > > On 8/27/21 10:18 AM, Borislav Petkov wrote: > > On Fri, Aug 20, 2021 at 10:19:27AM -0500, Brijesh Singh wrote: > >> From: Michael Roth <michael.roth@xxxxxxx> > >> > >> This adds support for utilizing the SEV-SNP-validated CPUID table in > > s/This adds support for utilizing/Utilize/ > > > > Yap, it can really be that simple. :) > > > >> the various #VC handler routines used throughout boot/run-time. Mostly > >> this is handled by re-using the CPUID lookup code introduced earlier > >> for the boot/compressed kernel, but at various stages of boot some work > >> needs to be done to ensure the CPUID table is set up and remains > >> accessible throughout. The following init routines are introduced to > >> handle this: > > Do not talk about what your patch does - that should hopefully be > > visible in the diff itself. Rather, talk about *why* you're doing what > > you're doing. > > > >> sev_snp_cpuid_init(): > > This one is not really introduced - it is already there. > > > > <snip all the complex rest> > > > > So this patch is making my head spin. It seems we're dancing a lot of > > dance just to have our CPUID page present at all times. Which begs the > > question: do we need it during the whole lifetime of the guest? > > Mike can correct me, we need it for entire lifetime of the guest. > Whenever guest needs the CPUID value, the #VC handler will refer to this > page. That's right, and cpuid instructions can get introduced at pretty much every stage of the boot process. > > > > Regardless, I think this can be simplified by orders of > > magnitude if we allocated statically 4K for that CPUID page in > > arch/x86/boot/compressed/mem_encrypt.S, copied the supplied CPUID page > > from the firmware to it and from now on, work with our own copy. > > Actually a VMM could populate more than one page for the CPUID. One > page can include 64 entries and I believe Mike is already running into > limits (with Qemu) and exploring the ideas to extend it more than a page. I added the range checks in this version so that a hypervisor can still leave out all-zero entries, so I think it can be avoided near-term at least, but yes, still a possibility we might need an extra one in the future, not sure how scarce storage is for stuff like __ro_after_init, so worth considering.
