2017-08-17 13:00+0800, Lan Tianyu: > On 2017年08月16日 21:26, Radim Krčmář wrote: > > 2017-08-15 21:58-0400, Lan Tianyu: > >> These CR3 VMEXITs was introduced for platform without "unrestricted guest" > >> support. This is to set ept identity table to guest CR3 in guest real > >> mode because these platforms don't support ept real mode(CR0.PE and CR0.PG > >> must be set to 1). But these VMEXITs is redundant for platforms with > >> "unrestricted guest" support. > >> > >> Signed-off-by: Lan Tianyu <[email protected]> > >> --- > >> arch/x86/kvm/vmx.c | 22 +++++++++++++--------- > >> 1 file changed, 13 insertions(+), 9 deletions(-) > >> > >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > >> @@ -4311,7 +4313,9 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, > >> unsigned long cr3) > >> } > >> > >> vmx_flush_tlb(vcpu); > >> - vmcs_writel(GUEST_CR3, guest_cr3); > >> + > >> + if (!enable_unrestricted_guest || !enable_ept) > >> + vmcs_writel(GUEST_CR3, guest_cr3); > > > > This looks wrong -- it would prevent update GUEST_CR3 outside of > > non-root mode with enable_unrestricted_guest. > > > > OK. Do you mean nest mode? I didn't consider that case. > I thought there were three cases here. > > 1) Shadow page mode(enable_ept=0) > > 2) ept mode without unrestricted guest mode > (ept=1, enable_unrestricted_guest = 0) > > 3) ept mode with unrestricted guest mode > (ept=1, enable_unrestricted_guest = 1) > > From my understanding, only (1) and (2) need to update guest cr3. > If nest mode is still needed to update guest CR3, we can add > is_guest_mode() in the if condition. Other choice is to just ignore > setting guest cr3 for case3. The condition maybe changed to
That too, but I was thinking about a more common (3) with enabled paging, where GUEST_CR3 should reflect what the guest wants there. Consider a case where the userspace changed CR3 (e.g. after migration), how would it get propagated to the guest? > if (!(enable_unrestricted_guest && enable_ept)) > vmcs_writel(GUEST_CR3, guest_cr3); It is the same. :) I would think that checking the condition is about as fast as doing the vmcs write, so we don't need to complicate the code.
