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SubjectRe: [PATCH 1/3] KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support
On Fri, Feb 28, 2020 at 06:59:43AM -0800, Sean Christopherson wrote:
> On Fri, Feb 28, 2020 at 11:16:10AM +0100, Paolo Bonzini wrote:
> > On 27/02/20 23:30, Sean Christopherson wrote:
> > > -void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs)
> > > +void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs, bool in_use)
> > > {
> > > vmcs_clear(loaded_vmcs->vmcs);
> > > if (loaded_vmcs->shadow_vmcs && loaded_vmcs->launched)
> > > vmcs_clear(loaded_vmcs->shadow_vmcs);
> > > +
> > > + if (in_use) {
> > > + list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
> > > +
> > > + /*
> > > + * Ensure deleting loaded_vmcs from its current percpu list
> > > + * completes before setting loaded_vmcs->vcpu to -1, otherwise
> > > + * a different cpu can see vcpu == -1 first and add loaded_vmcs
> > > + * to its percpu list before it's deleted from this cpu's list.
> > > + * Pairs with the smp_rmb() in vmx_vcpu_load_vmcs().
> > > + */
> > > + smp_wmb();
> > > + }
> > > +
> >
> > I'd like to avoid the new in_use argument and, also, I think it's a
> > little bit nicer to always invoke the memory barrier. Even though we
> > use "asm volatile" for vmclear and therefore the compiler is already
> > taken care of, in principle it's more correct to order the ->cpu write
> > against vmclear's.
>
> Completely agree on all points. I wanted to avoid in_use as well, but it
> didn't occur to me to use list_empty()...
>
> > This gives the following patch on top:
>
> Looks good.
>
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index c9d6152e7a4d..77a64110577b 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -656,25 +656,24 @@ static int vmx_set_guest_msr(struct vcpu_vmx *vmx, struct shared_msr_entry *msr,
> > return ret;
> > }
> >
> > -void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs, bool in_use)
> > +void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs)
> > {
> > vmcs_clear(loaded_vmcs->vmcs);
> > if (loaded_vmcs->shadow_vmcs && loaded_vmcs->launched)
> > vmcs_clear(loaded_vmcs->shadow_vmcs);
> >
> > - if (in_use) {
> > + if (!list_empty(&loaded_vmcs->loaded_vmcss_on_cpu_link))

Circling back to this, an even better option is to drop loaded_vmcs_init()
and open code the required pieces in __loaded_vmcs_clear() and
alloc_loaded_vmcs(). Those are the only two callers, and the latter
doesn't need to VMCLEAR the shadow VMCS (guaranteed to be NULL) and
obviously doesn't need the list manipulation of smp_wmb().

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