Re: [PATCH] KVM: PPC: Book3S HV: XIVE: Free previous EQ page when setting up a new one

[Greg Kurz <groug@kaod.org>]

On Mon, 11 Nov 2019 12:26:25 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> On 11/11/2019 10:49, Greg Kurz wrote:
> > The EQ page is allocated by the guest and then passed to the hypervisor
> > with the H_INT_SET_QUEUE_CONFIG hcall. A reference is taken on the page
> > before handing it over to the HW. This reference is dropped either when
> > the guest issues the H_INT_RESET hcall or when the KVM device is released.
> > But, the guest can legitimately call H_INT_SET_QUEUE_CONFIG several times
> > to reset the EQ (vCPU hot unplug) or set a new EQ (guest reboot). In both
> > cases the EQ page reference is leaked. This is especially visible when
> > the guest memory is backed with huge pages: start a VM up to the guest
> > userspace, either reboot it or unplug a vCPU, quit QEMU. The leak is
> > observed by comparing the value of HugePages_Free in /proc/meminfo before
> > and after the VM is run.
> > 
> > Note that the EQ reset path seems to be calling put_page() but this is
> > done after xive_native_configure_queue() which clears the qpage field
> > in the XIVE queue structure, ie. the put_page() block is a nop and the
> > previous page pointer was just overwritten anyway. In the other case of
> > configuring a new EQ page, nothing seems to be done to release the old
> > one.
> 
> Yes. Nice catch. I think we should try to fix the problem differently. 
> 
> The routine xive_native_configure_queue() is only suited for XIVE 
> drivers doing their own EQ page allocation: Linux PowerNV and the 
> KVM XICS-over-XIVE device. The KVM XIVE device acts as a proxy for 
> the guest OS doing the allocation and it has different needs.
> 

Well xive_native_configure_queue() is at least partially suited for all three
drivers since they use it to configure the EQ. But it doesn't address the page
allocation/de-allocation which is indeed different.

> Having a specific xive_native_configure_queue() for the KVM XIVE 
> device seems overkill. May be, we could introduce a helper routine 
> in KVM XIVE device calling xive_native_configure_queue() and handling 
> the page reference how it should be ? That is to drop the previous
> page reference in case of a change on q->qpage.
> 

Yes, that seems better. I'll post a v2 with the helper you've mailed
me.

> 
> Also, we should try to preserve the previous setting until the whole 
> configuration is in place. That seems possible up to the call to 
> xive_native_configure_queue(). If kvmppc_xive_attach_escalation()
> fails I think it is too late, as the HW has been configured by 
> xive_native_configure_queue(), and we should just cleanup everything. 
> 
> Thanks,
> 
> C. 
> 
> 
> > Fix both cases by always calling put_page() on the existing EQ page in
> > kvmppc_xive_native_set_queue_config(). This is a seemless change for the
> > EQ reset case. However this causes xive_native_configure_queue() to be
> > called twice for the new EQ page case: one time to reset the EQ and another
> > time to configure the new page. This is needed because we cannot release
> > the EQ page before calling xive_native_configure_queue() since it may still
> > be used by the HW. We cannot modify xive_native_configure_queue() to drop
> > the reference either because this function is also used by the XICS-on-XIVE
> > device which requires free_pages() instead of put_page(). This isn't a big
> > deal anyway since H_INT_SET_QUEUE_CONFIG isn't a hot path.
> > 
> > Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
> > Cc: stable@vger.kernel.org # v5.2
> > Fixes: 13ce3297c576 ("KVM: PPC: Book3S HV: XIVE: Add controls for the EQ configuration")
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  arch/powerpc/kvm/book3s_xive_native.c |   21 ++++++++++++---------
> >  1 file changed, 12 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
> > index 34bd123fa024..8ab908d23dc2 100644
> > --- a/arch/powerpc/kvm/book3s_xive_native.c
> > +++ b/arch/powerpc/kvm/book3s_xive_native.c
> > @@ -570,10 +570,12 @@ static int kvmppc_xive_native_set_queue_config(struct kvmppc_xive *xive,
> >  		 __func__, server, priority, kvm_eq.flags,
> >  		 kvm_eq.qshift, kvm_eq.qaddr, kvm_eq.qtoggle, kvm_eq.qindex);
> >  
> > -	/* reset queue and disable queueing */
> > -	if (!kvm_eq.qshift) {
> > -		q->guest_qaddr  = 0;
> > -		q->guest_qshift = 0;
> > +	/*
> > +	 * Reset queue and disable queueing. It will be re-enabled
> > +	 * later on if the guest is configuring a new EQ page.
> > +	 */
> > +	if (q->guest_qshift) {
> > +		page = virt_to_page(q->qpage);
> >  
> >  		rc = xive_native_configure_queue(xc->vp_id, q, priority,
> >  						 NULL, 0, true);
> > @@ -583,12 +585,13 @@ static int kvmppc_xive_native_set_queue_config(struct kvmppc_xive *xive,
> >  			return rc;
> >  		}
> >  
> > -		if (q->qpage) {
> > -			put_page(virt_to_page(q->qpage));
> > -			q->qpage = NULL;
> > -		}
> > +		put_page(page);
> >  
> > -		return 0;
> > +		if (!kvm_eq.qshift) {
> > +			q->guest_qaddr  = 0;
> > +			q->guest_qshift = 0;
> > +			return 0;
> > +		}
> >  	}
> >  
> >  	/*
> > 
>