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Re: [Qemu-arm] [PATCH V11 0/8] add pvpanic mmio support

From: Michael S. Tsirkin
Subject: Re: [Qemu-arm] [PATCH V11 0/8] add pvpanic mmio support
Date: Tue, 4 Dec 2018 14:10:35 -0500

On Tue, Dec 04, 2018 at 02:53:26PM +0100, Paolo Bonzini wrote:
> On 04/12/18 14:43, Peter Maydell wrote:
> > The point about PCI is that it is the same everywhere and
> > discoverable, and easy for the user to add to the system or not.
> > MMIO requires extra work for every board model that we want to
> > put the device into, plus extra on both kernel and QEMU side
> > for every system description mechanism (ACPI, dtb, whatever
> > some future architecture might use), even if we have the basic
> > "mmio pvpanic" device code already.
> Looks like dtb is becoming a standard?  Even RISC-V switched from their
> own system description to device tree.  Anyway, this is not too
> important.  I agree with you about discoverability, on the other hand if
> we could have something defined by the vendor rather than QEMU it would
> be even better.  (Even better would be something that distro kernels
> already have support for, but that would be asking too much probably).
> >> Also, while reusing code in general is nice, sometimes there are
> >> platform-specific ways to do it.  For ARM, for example, would it make
> >> sense to use an HVC/SMC that "extends" the PSCI, and pass the number in
> >> the PSCI device tree node?
> > 
> > If you want a hypercall then these days the arm HVC calling convention
> > includes mechanisms for discoverably determining whether a particular
> > hypercall is supported, so you wouldn't need to pass anything in the
> > ACPI or dtb. But I didn't get the impression that anybody wanted a
> > hypercall for this particularly.
> Not for x86, where each hypervisor has its own hypercall number and even
> its calling convention.  But for ARM it already makes more sense.
> >> Related to this, is there a more or less "standard" watchdog device on
> >> ARM that could be added to virt?  There is the SBSA watchdog, but it's
> >> ugly for implementation in KVM because it counts down with frequency
> >> equal to CNTFRQ (which I'm not sure if QEMU has access too, and also it
> >> doesn't play well with live migration).
> > 
> > The i6300esb is PCI, presumably that would work?
> Yeah, I was wondering if there was something in PrimeCell.  I found
> SP805 exists now.
> >>> I notice also that there's a mention in that thread that the pvpanic
> >>> ACPI table entry on x86 resulted in unhelpful Windows notifications
> >>> about new devices it didn't understand. Is that going to be an issue
> >>> for Arm with this mmio pvpanic ?
> >>
> >> Yes, it is probably the same as for x86.
> > 
> > I guess we need to find out if that is a problem before we can
> > merge this, then.
> As long as the pvpanic device is not added by default it's okay.
> Paolo

It isn't too bad, you just include a driver and then it's happy.

Isn't a device just for panic going too far though?
Should it be some kind of PV device that we
can use to add more pv stuff in the future, with
a panic option?


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