Messages in this thread | | | Date | Mon, 06 Dec 2010 19:01:48 +0200 | From | Avi Kivity <> | Subject | Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices |
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On 12/06/2010 06:46 PM, Jan Kiszka wrote: > Am 06.12.2010 17:40, Avi Kivity wrote: > > On 12/06/2010 06:34 PM, Jan Kiszka wrote: > >>> > >>> What's the protocol for doing this? I suppose userspace has to disable > >>> interrupts, ioctl(SET_INTX_MASK, masked), ..., ioctl(SET_INTX_MASK, > >>> unmasked), enable interrupts? > >> > >> Userspace just has to synchronize against itself - what it already does: > >> qemu_mutex, and masking/unmasking is synchronous /wrt the the executing > >> VCPU. Otherwise, masking/unmasking is naturally racy, also in Real Life. > >> The guest resolves the remaining races. > > > > I meant when qemu sets INTX_MASK and the kernel clears it immediately > > afterwards because the two are not synchronized. I guess that won't > > happen in practice because playing with INTX_MASK is very rare. > > Ah, there is indeed a race, and the qemu-kvm patches I did not post yet > (to wait for the kernel interface to settle) actually suffer from it: > userspace needs to set the kernel mask before writing the config space > (it's the other way around ATM). This avoids that the kernel overwrites > what userspace just wrote out. We always suffer from the race the other > way around, see below.
Please document the protocol. Is this always the right order? Shouldn't it be reversed when unmasking? I admit I'm confused about this.
> >> > >> I think this is what VFIO does and is surely cleaner than this approach. > >> But it's not possible with the existing interface (sysfs + KVM ioctls) - > >> or can you restrict the sysfs access to the config space in such details? > > > > I'm sure you can, not sure it's worth it. Can the situation be > > exploited? what if userspace lies? > > That's also the above scenario inverted: Userspace can mask or unmask at > any time. If it unmasks a yet unhandled, thus raise interrupt, it will > trigger another one. The kernel will catch it and mask it again. That > can repeat forever with the frequency userspace is able to run its > unmasking code. Not nice, but nothing to leverage for a DoS.
Ok (I think).
-- error compiling committee.c: too many arguments to function
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