On 02/03/2018 11:55, Vitaly Kuznetsov wrote: > Some time ago Paolo suggested to take a look at probably unneeded expensive > rdmsrs for FS/GS base MSR in vmx_save_host_state(). This is called on every > vcpu run when we need to handle vmexit in userspace. > > I have to admit I got a bit lost in our kernel FS/GS magic. I managed to > convince myself that in the well defined context (ioctl from userspace) > we can always get the required values from in-kernel variables and avoid > rdmsrs. But I may have missed something really important, thus RFC. > > My debug shows we're shaving off 240 cpu cycles (E5-2603 v3). > > In case these patches turn out to be worthwile AMD SVM can probably be > optimized the ame way. SVM is a bit different, because it uses VMLOAD/VMSAVE and so it doesn't have an equivalent of vmx_save_host_state(). Unfortunately, you cannot really eliminate VMLOAD/VMSAVE because it's the only way to load the hidden state of TR and LDTR---so you might as well use it to load FS and GS, even in 64-bit mode. In order to decrease the cost of vmload/vmsave, we could single out the simplest vmexit handlers and process them without even getting out of svm_vcpu_run, thus skipping all four of stgi/vmload/vmsave/clgi. However, this probably couldn't be done for the really common vmexits such as nested page fault, PIO or most MSR accesses. We _could_ do it for nested virt-related vmexits, but the advantage of that is getting smaller too, since Zen provides hardware support for nested GIF and nested VMLOAD/VMSAVE. Paolo > Vitaly Kuznetsov (3): > x86/kvm/vmx: read MSR_FS_BASE from current->thread > x86/kvm/vmx: read MSR_KERNEL_GS_BASE from current->thread > x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE > > arch/x86/kernel/cpu/common.c | 1 + > arch/x86/kvm/vmx.c | 7 ++++--- > 2 files changed, 5 insertions(+), 3 deletions(-) >
