linux-kernel - Re: [PATCH v4 09/11] KVM: arm/arm64: vgic-its: free caches when GITS

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Date:   Sat, 21 Oct 2017 17:42:34 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        marc.zyngier@....com, peter.maydell@...aro.org,
        andre.przywara@....com, wanghaibin.wang@...wei.com,
        wu.wubin@...wei.com, drjones@...hat.com, wei@...hat.com
Subject: Re: [PATCH v4 09/11] KVM: arm/arm64: vgic-its: free caches when
 GITS_BASER Valid bit is cleared

On Sat, Oct 21, 2017 at 04:36:22PM +0200, Auger Eric wrote:
> Hi Christoffer,
> 
> On 21/10/2017 16:31, Christoffer Dall wrote:
> > On Sat, Oct 21, 2017 at 12:13:21PM +0200, Auger Eric wrote:
> >> Hi Christoffer,
> >>
> >> On 18/10/2017 00:34, Christoffer Dall wrote:
> >>> On Tue, Oct 17, 2017 at 09:10:07AM +0200, Eric Auger wrote:
> >>>> When the GITS_BASER<n>.Valid gets cleared, the data structures in
> >>>> guest RAM are not valid anymore. The device, collection
> >>>> and LPI lists stored in the in-kernel ITS represent the same
> >>>> information in some form of cache. So let's void the cache.
> >>>>
> >>>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> >>>>
> >>>> ---
> >>>>
> >>>> v2 -> v3:
> >>>> - add a comment and clear cache in if block
> >>>> ---
> >>>>  virt/kvm/arm/vgic/vgic-its.c | 23 +++++++++++++++++++----
> >>>>  1 file changed, 19 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> >>>> index f3f0026f..084239c 100644
> >>>> --- a/virt/kvm/arm/vgic/vgic-its.c
> >>>> +++ b/virt/kvm/arm/vgic/vgic-its.c
> >>>> @@ -1471,8 +1471,9 @@ static void vgic_mmio_write_its_baser(struct kvm *kvm,
> >>>>  				      unsigned long val)
> >>>>  {
> >>>>  	const struct vgic_its_abi *abi = vgic_its_get_abi(its);
> >>>> -	u64 entry_size, device_type;
> >>>> +	u64 entry_size;
> >>>>  	u64 reg, *regptr, clearbits = 0;
> >>>> +	int type;
> >>>>  
> >>>>  	/* When GITS_CTLR.Enable is 1, we ignore write accesses. */
> >>>>  	if (its->enabled)
> >>>> @@ -1482,12 +1483,12 @@ static void vgic_mmio_write_its_baser(struct kvm *kvm,
> >>>>  	case 0:
> >>>>  		regptr = &its->baser_device_table;
> >>>>  		entry_size = abi->dte_esz;
> >>>> -		device_type = GITS_BASER_TYPE_DEVICE;
> >>>> +		type = GITS_BASER_TYPE_DEVICE;
> >>>>  		break;
> >>>>  	case 1:
> >>>>  		regptr = &its->baser_coll_table;
> >>>>  		entry_size = abi->cte_esz;
> >>>> -		device_type = GITS_BASER_TYPE_COLLECTION;
> >>>> +		type = GITS_BASER_TYPE_COLLECTION;
> >>>>  		clearbits = GITS_BASER_INDIRECT;
> >>>>  		break;
> >>>>  	default:
> >>>> @@ -1499,10 +1500,24 @@ static void vgic_mmio_write_its_baser(struct kvm *kvm,
> >>>>  	reg &= ~clearbits;
> >>>>  
> >>>>  	reg |= (entry_size - 1) << GITS_BASER_ENTRY_SIZE_SHIFT;
> >>>> -	reg |= device_type << GITS_BASER_TYPE_SHIFT;
> >>>> +	reg |= (u64)type << GITS_BASER_TYPE_SHIFT;
> >>>>  	reg = vgic_sanitise_its_baser(reg);
> >>>>  
> >>>>  	*regptr = reg;
> >>>> +
> >>>> +	/* Table no longer valid: clear cached data */
> >>>> +	if (!(reg & GITS_BASER_VALID)) {
> >>>> +		switch (type) {
> >>>> +		case GITS_BASER_TYPE_DEVICE:
> >>>> +			vgic_its_free_device_list(kvm, its);
> >>>> +			break;
> >>>> +		case GITS_BASER_TYPE_COLLECTION:
> >>>> +			vgic_its_free_collection_list(kvm, its);
> >>>> +			break;
> >>>> +		default:
> >>>> +			break;
> >>>> +		}
> >>>> +	}
> >>>
> >>> So we do this after setting the *regptr, which makes we worried about
> >>> races.
> >>>
> >>> How are guest writes to these registers synchronized with, for example
> >>> trying to save the tables.  Perhaps we don't care because userspace
> >>> should have stopped the VM before trying to save the ITS state?
> >>
> >> Yes save & restore can happen only if all vcpus are locked. Same for
> >> user space accesses.
> >>
> > 
> > So this cannot happens, because the save/restore operation will fail to
> > get the lock of an executing VCPU which is modifying this function?
> 
> Yes that's my understanding.
> 

ok.  If you respin, putting a comment here may be worth it.

Thanks,
-Christoffer