KVM: VMX: Avoid rearranging switched guest msrs while they are loaded

KVM tries to run as much as possible with the guest msrs loaded instead of
host msrs, since switching msrs is very expensive.  It also tries to minimize
the number of msrs switched according to the guest mode; for example,
MSR_LSTAR is needed only by long mode guests.  This optimization is done by
setup_msrs().

However, we must not change which msrs are switched while we are running with
guest msr state:

 - switch to guest msr state
 - call setup_msrs(), removing some msrs from the list
 - switch to host msr state, leaving a few guest msrs loaded

An easy way to trigger this is to kexec an x86_64 linux guest.  Early during
setup, the guest will switch EFER to not include SCE.  KVM will stop saving
MSR_LSTAR, and on the next msr switch it will leave the guest LSTAR loaded.
The next host syscall will end up in a random location in the kernel.

Fix by reloading the host msrs before changing the msr list.

Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 61c2a3a8d20a9635da6c0d2d48705dba84ac1156..94ea724638fda63b87ec248c2c3b40783ad93e08 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -638,6 +638,7 @@ static void setup_msrs(struct vcpu_vmx *vmx)
 {
        int save_nmsrs;
 
+       vmx_load_host_state(vmx);
        save_nmsrs = 0;
 #ifdef CONFIG_X86_64
        if (is_long_mode(&vmx->vcpu)) {