On 09/06/2017 03:13, 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 Not sure I understand this completely, but: /* Get the fixmap index for a specific processor */ static inline unsigned int get_cpu_gdt_ro_index(int cpu) { - return FIX_GDT_REMAP_BEGIN + cpu; + return FIX_GDT_REMAP_END - cpu * PAGES_PER_GDT; } isn't this off by one. I think it should be FIX_GDT_REMAP_END + 1 - cpu * PAGES_PER_GDT or just FIX_GDT_REMAP_BEGIN + cpu * PAGES_PER_GDT? That is for example: FIX_GDT_REMAP_BEGIN = 100 get_cpu_gdt_ro_index(0) = 100 get_cpu_gdt_ro_index(1) = 116 get_cpu_gdt_ro_index(2) = 132 get_cpu_gdt_ro_index(3) = 148 FIX_GDT_REMAP_END = 163 Paolo
