On 09/11/2012 12:35 PM, Gleb Natapov wrote:
> On Tue, Sep 11, 2012 at 12:29:06PM +0300, Avi Kivity wrote:
>> On 09/10/2012 08:05 PM, Gleb Natapov wrote:
>> > On Mon, Sep 10, 2012 at 07:17:54PM +0300, Gleb Natapov wrote:
>> >> > > + return 0;
>> >> > > +}
>> >> > > +
>> >> > > +static inline int kvm_apic_set_id(struct kvm_lapic *apic, u8 id)
>> >> > > +{
>> >> > > + apic_set_reg(apic, APIC_ID, id << 24);
>> >> > > + return recalculate_apic_map(apic->vcpu->kvm);
>> >> > > +}
>> >> > > +
>> >> > > +static inline int kvm_apic_set_ldr(struct kvm_lapic *apic, u32 id)
>> >> > > +{
>> >> > > + apic_set_reg(apic, APIC_LDR, id);
>> >> > > + return recalculate_apic_map(apic->vcpu->kvm);
>> >> > > +}
>> >> > > +
>> >> >
>> >> > return value of these functions seems never checked.
>> >> >
>> >> Yes, the problem is that we can do nothing about the failure if failure
>> >> happens during guest write.
>>
>> We can. Return -ENOMEM all the way up to userspace.
>>
> There is no userspace to return error to if error happens on guest MMIO
> write. Unless you mean return it as a return value of ioctl(VM_RUN) in
> which case it is equivalent of killing the guest.
That is what I meant.
> And this is not fair
> to a guest who did nothing wrong to suffer from our stupid optimizations :)
> Actually I am not sure that returning to userspace in the middle of an
> IO that is handled by a kernel is well defined in KVM ABI.
If you get -ENOMEM when allocating a page without GFP_ATOMIC (or
GFP_NOIO etc) then the entire host is dead anyway. The same thing can
happen if the guest (or userspace) touches a yet-unallocated page, or if
the page fault path fails to allocate mmu pages, or any of a thousand
other allocations we have all over.
>
>> >>
>> > Actually I have an idea how to handle the error. Never return one. If
>> > map cannot be allocated go slow path always. phys_map should be checked
>> > for NULL during delivery in this case obviously.
>>
>> That's better of course (though we have to beware of such tricks, but in
>> this case the slow path is regularly exercised so it should keep working).
>>
> Oh with Windows guests it has work to do for sure.
This reminds me, we could speed up self-ipi for that.
--
error compiling committee.c: too many arguments to function
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