linux-kernel - Re: Hyper-V: Question about initializing hypercall interface

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Date:   Mon, 30 May 2022 07:56:25 +0200
From:   Vit Kabele <vit.kabele@...go.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "rudolf.marek@...go.com" <rudolf.marek@...go.com>,
        "vit@...ele.me" <vit@...ele.me>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
Subject: Re: 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