On Mon, Feb 20, 2017 at 3:05 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > On 18/02/2017 04:29, Andy Lutomirski wrote: >> There's no code here because the patch is trivial, but I want to run >> the idea by you all first to see if there are any issues. >> >> VMX is silly and forces the TSS limit to the minimum on VM exits. KVM >> wastes lots of cycles bumping it back up to accomodate the io bitmap. > > Actually looked at the code now... > > reload_tss is only invoked for userspace exits, so it is a nice-to-have > but it wouldn't show on most workloads. Still it does save 150-200 > clock cycles to remove it (I just commented out reload_tss() from > __vmx_load_host_state to test). That's for anything involving userspace or preemption, right? > > Another 100-150 could be saved if we could just use rdgsbase/wrgsbase, > instead of rdmsr/wrmsr, to read and write the kernel GS. Super hacky > patch after sig. I have a Real Patch Series (tm) to do that, but it has a couple of unresolved corner cases so far. That being said, vmx_save_host_state() is, um, poorly optimized. I'll try to find some time to fix the obvious things. Meanwhile, I'll send real patches for TR. > + cr4_set_bits(X86_CR4_FSGSBASE); Nice root hole :-p --Andy
