Re: [Xen-devel] [PATCH] x86/vm_event: correctly gather gs_shadow value

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, May 1, 2019 at 8:53 AM Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>
> On Wed, May 1, 2019 at 8:20 AM Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> >
> > On 5/1/19 4:58 PM, Tamas K Lengyel wrote:
> > >>> It might be worth introducing a "sync state from hw" hook which collects
> > >>> all the data we intend to pass to the introspection agent.
> > >>
> > >> You mean adding another hvm hook?
> > >
> > > Actually, instead of another hook I think what would make sense it to
> > > just update vmx_save_vmcs_ctxt to automatically refresh the cached
> > > register values when it's called with "v == current". Thoughts?
> >
> > That's probably the better way to go about it, since otherwise the
> > xc_hvm_getcontext_partial() hypercall will suffer from the same problem.
> > (there are two ways of getting guest state: one is via the vm_event
> > cached values, the other is via the aforementioned hypercall).
>
> True, although issuing the hypercall in the vm_event callback is
> actually fine - that's how I found the issue to begin with, since the
> vCPU will be scheduled out with the cached registers refreshed and
> thus be different then what the vm_event itself had. But other callers
> of the hypercall can run into the problem if the guest/vcpu is not
> paused.

Actually, doing the "v == current" check wouldn't really do anything
for the hypercall since it's not the domain issuing the hypercall on
itself. The only way to ensure that hypercall is returning correct
values would be to pause the vCPU.

Tamas

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