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SubjectRe: Hyper-V: Question about initializing hypercall interface

On Tue, May 24, 2022 at 05:43:33AM +0000, Michael Kelley (LINUX) wrote:
> > >
> > > 2/ The Lock bit (bit 1) is ignored in the Linux implementation. If the
> > > hypervisor starts with Lock bit set, the init function allocates the
> > > hv_hypercall_pg and writes the value to the MSR, then:
> > > a/ If the hypervisor ignores the write, the MSR remains unchanged,
> > > but the global variable is already set. Attempt to do a
> > > hypercall ends with call to undefined memory, because the code
> > > in hv_do_hypercall() checks the global variable against NULL,
> > > which will pass.
> > > b/ The hypervisor injects #GP, in which case the guest crashes.
> >
> > I would need to confirm with the Hyper-V team, but I think the Lock
> > bit would only be set *after* the guest OS has provided a guest page
> > to be used as the hypercall page.
> >
> > There is code in Linux to clear the MSR and disable the hypercall
> > page when doing a kexec or kdump. This is done so that the new
> > kernel can start "fresh" and establish its own hypercall page. That
> > kexec/kdump code does not check the Lock bit, and I'm not sure of
> > the implications if the Lock bit were found to be set in such a case.
> >
> > I'll check with the Hyper-V team to get clarity on the handling
> > of the Lock bit in the case of trying to disable the hypercall page.
> >
> > Michael
>
> The Hyper-V team clarified that the Locked bit is never set by
> the hypervisor. The bit is there for the guest to set if it chooses.
> The TLFS is indeed not clear on this point.
Thank you for the clarification. I'll update our implementation
accordingly :)

--
Best regards,
Vit Kabele

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