Re: [Qemu-devel] 3.1.0-rc{0,1} doesn't start

[Eduardo Habkost]

On Tue, Nov 20, 2018 at 07:40:01PM +0100, Paolo Bonzini wrote:
> On 20/11/18 00:00, Eduardo Habkost wrote:
> > On Mon, Nov 19, 2018 at 04:55:13PM -0500, Bandan Das wrote:
> >> address@hidden writes:
> >>
> >>> hello
> >>>
> >>> I'm building qemu from source and happily using it since a bit
> >>> (2.3.0)
> >>>
> >>> Since 3.1.0-rc0 (including latest 3.1.0-rc1) I'm no more able to start
> >>> qemu, getting:
> >>>
> >>>     ----8<----
> >>>     install:115> qemu
> >>>     qemu: error: failed to set MSR 0x10a to 0x0
> >>>     qemu: 
> >>> /home/balducci/tmp/install-us-d/qemu-3.1.0-rc1.d/qemu-3.1.0-rc0/target/i386/kvm.c:2185:
> >>>  kvm_put_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
> >>>     Aborted
> >>>     ---->8----
> >>>
> >> I believe the check on whether MSR_IA32_ARCH_CAPABILITIES is present is
> >> incomplete because it can return 0 for data. Can you try this:
> >>
> >> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> >> index f524e7d929..4878ffb90b 100644
> >> --- a/target/i386/kvm.c
> >> +++ b/target/i386/kvm.c
> >> @@ -2002,14 +2002,9 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
> >>  #endif
> >>  
> >>      /* If host supports feature MSR, write down. */
> >> -    if (kvm_feature_msrs) {
> >> -        int i;
> >> -        for (i = 0; i < kvm_feature_msrs->nmsrs; i++)
> >> -            if (kvm_feature_msrs->indices[i] == 
> >> MSR_IA32_ARCH_CAPABILITIES) {
> >> -                kvm_msr_entry_add(cpu, MSR_IA32_ARCH_CAPABILITIES,
> >> +    if (kvm_arch_get_supported_msr_feature(kvm_state, 
> >> MSR_IA32_ARCH_CAPABILITIES)) {
> >> +        kvm_msr_entry_add(cpu, MSR_IA32_ARCH_CAPABILITIES,
> >>                                env->features[FEAT_ARCH_CAPABILITIES]);
> > 
> > kvm_arch_get_supported_msr_feature() will return the value of the
> > MSR on the host side (kvm/x86.c:kvm_get_msr_feature()).  Having
> > it return non-zero doesn't mean KVM's
> > svm_set_msr(MSR_IA32_ARCH_CAPABILITIES) will work.
> > 
> > If the MSR doesn't work on KVM_SET_MSRS, it is not supposed to
> > appear on KVM_GET_MSR_INDEX_LIST (even if it appears on
> > KVM_GET_MSR_FEATURE_INDEX_LIST).  QEMU must check
> > KVM_GET_MSR_INDEX_LIST too before including the MSR on the
> > KVM_SET_MSRS call.
> 
> Yes, this is a KVM bug.  For 3.1, making it "writable if nonzero" is a
> valid workaround, because AMD processors always return 0. It's not the
> prettiest thing, but it works.

So it looks like we have 2 bugs?  Is KVM incorrectly returning
the MSR on KVM_GET_MSR_INDEX_LIST?

I thought the bug was only in QEMU, which is supposed to be
checking KVM_GET_MSR_INDEX_LIST before setting the MSR (it is
only checking KVM_GET_MSR_FEATURE_INDEX_LIST).

-- 
Eduardo