On 09/06/2017 02:13, Andy Lutomirski wrote: > Hi all- > > As promised when Thomas did his GDT fixmap work, here is a draft patch > to speed up KVM by extending it. > > The downside of this patch is that it makes the fixmap significantly > larger on 64-bit systems if NR_CPUS is large (it adds 15 more pages > per CPU). I don't know if we care at all. It also bloats the kernel > image by 4k and wastes 4k of RAM for the entire time the system is > booted. We could avoid the latter bit (sort of) by not mapping the > extra fixmap pages at all and handling the resulting faults somehow. > That would scare me -- now we have IRET generating #PF when running > malicious , and that way lies utter madness. > > The upside is that we don't need to do LGDT after a vmexit on VMX. > LGDT is slooooooooooow. But no, I haven't benchmarked this yet. > > What do you all think? > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/kvm&id=e249a09787d6956b52d8260b2326d8f12f768799 > > Andrew/Boris/Juergen: what does Xen think about setting a very high > GDT limit? Will it let us? Should I fix it by changing > load_fixmap_gdt() (i.e. uncommenting the commented bit) or by teaching > the Xen paravirt code to just ignore the monstrous limit? Or is it > not a problem in the first place? When running PV, any selector under 0xe000 is fair game, and anything over that is Xen's. OTOH, the set of software running as a PV guest, and also running KVM is empty. An HVM guest (which when nested, is the only viable option to run KVM) has total control over its GDT. ~Andrew
