Subject: [PATCH] ARM64: KVM: Fix coherent_icache_guest_page() for host with external L3-cache.

From: [email-masked] (Christoffer Dall)
Date: Fri, 16 Aug 2013 10:28:06 -0700

On Fri, Aug 16, 2013 at 10:48:39PM +0530, Anup Patel wrote:
> On 16 August 2013 22:44, Christoffer Dall <[email-masked]>wrote:
> 
> > On Fri, Aug 16, 2013 at 10:32:30AM +0530, Anup Patel wrote:
> > > On Thu, Aug 15, 2013 at 10:23 PM, Christoffer Dall
> > > <[email-masked]> wrote:
> > > > On Thu, Aug 15, 2013 at 04:37:27PM +0100, Marc Zyngier wrote:
> > > >> On 2013-08-15 16:13, Anup Patel wrote:
> > > >> > Hi Marc,
> > > >> >
> > > >> > On Thu, Aug 15, 2013 at 8:17 PM, Marc Zyngier <[email-masked]
> > >
> > > >> > wrote:
> > > >> >> On 2013-08-15 14:31, Anup Patel wrote:
> > > >> >>>
> > > >> >>> On Thu, Aug 15, 2013 at 2:01 PM, Marc Zyngier
> > > >> >>> <[email-masked]>
> > > >> >>> wrote:
> > > >> >>>>
> > > >> >>>> On 2013-08-15 07:26, Anup Patel wrote:
> > > >> >>>>>
> > > >> >>>>>
> > > >> >>>>> Hi Marc,
> > > >> >>>>>
> > > >> >>>>> On Thu, Aug 15, 2013 at 10:22 AM, Marc Zyngier
> > > >> >>>>> <[email-masked]>
> > > >> >>>>> wrote:
> > > >> >>>>>>
> > > >> >>>>>>
> > > >> >>>>>> Hi Anup,
> > > >> >>>>>>
> > > >> >>>>>>
> > > >> >>>>>> On 2013-08-14 15:22, Anup Patel wrote:
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>> On Wed, Aug 14, 2013 at 5:34 PM, Marc Zyngier
> > > >> >>>>>>> <[email-masked]>
> > > >> >>>>>>> wrote:
> > > >> >>>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>> Hi Pranav,
> > > >> >>>>>>>>
> > > >> >>>>>>>> On 2013-08-14 12:47, Pranavkumar Sawargaonkar wrote:
> > > >> >>>>>>>>>
> > > >> >>>>>>>>>
> > > >> >>>>>>>>>
> > > >> >>>>>>>>> Systems with large external L3-cache (few MBs), might have
> > > >> >>>>>>>>> dirty
> > > >> >>>>>>>>> content belonging to the guest page in L3-cache. To tackle
> > > >> >>>>>>>>> this,
> > > >> >>>>>>>>> we need to flush such dirty content from d-cache so that
> > > >> >>>>>>>>> guest
> > > >> >>>>>>>>> will see correct contents of guest page when guest MMU is
> > > >> >>>>>>>>> disabled.
> > > >> >>>>>>>>>
> > > >> >>>>>>>>> The patch fixes coherent_icache_guest_page() for external
> > > >> >>>>>>>>> L3-cache.
> > > >> >>>>>>>>>
> > > >> >>>>>>>>> Signed-off-by: Pranavkumar Sawargaonkar
> > > >> >>>>>>>>> <[email-masked]>
> > > >> >>>>>>>>> Signed-off-by: Anup Patel <[email-masked]>
> > > >> >>>>>>>>> ---
> > > >> >>>>>>>>>  arch/arm64/include/asm/kvm_mmu.h |    2 ++
> > > >> >>>>>>>>>  1 file changed, 2 insertions(+)
> > > >> >>>>>>>>>
> > > >> >>>>>>>>> diff --git a/arch/arm64/include/asm/kvm_mmu.h
> > > >> >>>>>>>>> b/arch/arm64/include/asm/kvm_mmu.h
> > > >> >>>>>>>>> index efe609c..5129038 100644
> > > >> >>>>>>>>> --- a/arch/arm64/include/asm/kvm_mmu.h
> > > >> >>>>>>>>> +++ b/arch/arm64/include/asm/kvm_mmu.h
> > > >> >>>>>>>>> @@ -123,6 +123,8 @@ static inline void
> > > >> >>>>>>>>> coherent_icache_guest_page(struct kvm *kvm, gfn_t gfn)
> > > >> >>>>>>>>>       if (!icache_is_aliasing()) {            /* PIPT */
> > > >> >>>>>>>>>               unsigned long hva = gfn_to_hva(kvm, gfn);
> > > >> >>>>>>>>>               flush_icache_range(hva, hva + PAGE_SIZE);
> > > >> >>>>>>>>> +             /* Flush d-cache for systems with external
> > > >> >>>>>>>>> caches. */
> > > >> >>>>>>>>> +             __flush_dcache_area((void *) hva, PAGE_SIZE);
> > > >> >>>>>>>>>       } else if (!icache_is_aivivt()) {       /* non
> > > >> >>>>>>>>> ASID-tagged
> > > >> >>>>>>>>> VIVT
> > > >> >>>>>>>>> */
> > > >> >>>>>>>>>               /* any kind of VIPT cache */
> > > >> >>>>>>>>>               __flush_icache_all();
> > > >> >>>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>> [adding Will to the discussion as we talked about this in the
> > > >> >>>>>>>> past]
> > > >> >>>>>>>>
> > > >> >>>>>>>> That's definitely an issue, but I'm not sure the fix is to
> > hit
> > > >> >>>>>>>> the
> > > >> >>>>>>>> data
> > > >> >>>>>>>> cache on each page mapping. It looks overkill.
> > > >> >>>>>>>>
> > > >> >>>>>>>> Wouldn't it be enough to let userspace do the cache cleaning?
> > > >> >>>>>>>> kvmtools
> > > >> >>>>>>>> knows which bits of the guest memory have been touched, and
> > > >> >>>>>>>> can do a
> > > >> >>>>>>>> "DC
> > > >> >>>>>>>> DVAC" on this region.
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>> It seems a bit unnatural to have cache cleaning is user-space.
> > > >> >>>>>>> I am
> > > >> >>>>>>> sure
> > > >> >>>>>>> other architectures don't have such cache cleaning in
> > > >> >>>>>>> user-space for
> > > >> >>>>>>> KVM.
> > > >> >>>>>>>
> > > >> >>>>>>>>
> > > >> >>>>>>>> The alternative is do it in the kernel before running any
> > vcpu
> > > >> >>>>>>>> - but
> > > >> >>>>>>>> that's not very nice either (have to clean the whole of the
> > > >> >>>>>>>> guest
> > > >> >>>>>>>> memory, which makes a full dcache clean more appealing).
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>>
> > > >> >>>>>>> Actually, cleaning full d-cache by set/way upon first run of
> > > >> >>>>>>> VCPU was
> > > >> >>>>>>> our second option but current approach seemed very simple
> > hence
> > > >> >>>>>>> we went for this.
> > > >> >>>>>>>
> > > >> >>>>>>> If more people vote for full d-cache clean upon first run of
> > > >> >>>>>>> VCPU then
> > > >> >>>>>>> we should revise this patch.
> > > >> >>>>>>
> > > >> >>>>>>
> > > >> >>>>>>
> > > >> >>>>>>
> > > >> >>>>>> Can you please give the attached patch a spin on your HW? I've
> > > >> >>>>>> boot-tested
> > > >> >>>>>> it on a model, but of course I can't really verify that it
> > fixes
> > > >> >>>>>> your
> > > >> >>>>>> issue.
> > > >> >>>>>>
> > > >> >>>>>> As far as I can see, it should fix it without any additional
> > > >> >>>>>> flushing.
> > > >> >>>>>>
> > > >> >>>>>> Please let me know how it goes.
> > > >> >>>>>
> > > >> >>>>>
> > > >> >>>>>
> > > >> >>>>> HCR_EL2.DC=1 means all memory access for Stage1 MMU off are
> > > >> >>>>> treated as "Normal Non-shareable, Inner Write-Back
> > > >> >>>>> Write-Allocate,
> > > >> >>>>> Outer Write-Back Write-Allocate memory"
> > > >> >>>>>
> > > >> >>>>> HCR_EL2.DC=0 means all memory access for Stage1 MMU off are
> > > >> >>>>> treated as "Strongly-ordered device memory"
> > > >> >>>>>
> > > >> >>>>> Now if Guest/VM access hardware MMIO devices directly (such as
> > > >> >>>>> VGIC CPU interface) with MMU off then MMIO devices will be
> > > >> >>>>> accessed as normal memory when HCR_EL2.DC=1.
> > > >> >>>>
> > > >> >>>>
> > > >> >>>>
> > > >> >>>> I don't think so. Stage-2 still applies, and should force MMIO to
> > > >> >>>> be
> > > >> >>>> accessed as device memory.
> > > >> >>>>
> > > >> >>>>
> > > >> >>>>> The HCR_EL2.DC=1 makes sense only if we have all software
> > > >> >>>>> emulated devices for Guest/VM which is not true for KVM ARM or
> > > >> >>>>> KVM ARM64 because we use VGIC.
> > > >> >>>>>
> > > >> >>>>> IMHO, this patch enforces incorrect memory attribute for
> > Guest/VM
> > > >> >>>>> when Stage1 MMU is off.
> > > >> >>>>
> > > >> >>>>
> > > >> >>>>
> > > >> >>>> See above. My understanding is that HCR.DC controls the default
> > > >> >>>> output of
> > > >> >>>> Stage-1, and Stage-2 overrides still apply.
> > > >> >>>
> > > >> >>>
> > > >> >>> You had mentioned that PAGE_S2_DEVICE attribute was redundant
> > > >> >>> and wanted guest to decide the memory attribute. In other words,
> > > >> >>> you
> > > >> >>> did not want to enforce any memory attribute in Stage2.
> > > >> >>>
> > > >> >>> Please refer to this patch
> > > >> >>> https://patchwork.kernel.org/patch/2543201/
> > > >> >>
> > > >> >>
> > > >> >> This patch has never been merged. If you carry on following the
> > > >> >> discussion,
> > > >> >> you will certainly notice it was dropped for a very good reason:
> > > >> >>
> > https://lists.cs.columbia.edu/pipermail/kvmarm/2013-May/005827.html
> > > >> >>
> > > >> >> So Stage-2 memory attributes are used, they are not going away, and
> > > >> >> they are
> > > >> >> essential to the patch I sent this morning.
> > > >> >>
> > > >> >>
> > > >> >>         M.
> > > >> >> --
> > > >> >> Fast, cheap, reliable. Pick two.
> > > >> >
> > > >> > HCR_EL2.DC=1 will break Guest/VM bootloader/firmware if the Guest/VM
> > > >> > is
> > > >> > provided a DMA-capable device in pass-through mode. The reason being
> > > >> > bootloader/firmware typically don't enable MMU and such
> > > >> > bootloader/firmware
> > > >> > will programme a pass-through DMA-capable device without any flushes
> > > >> > to
> > > >> > guest RAM (because it has not enabled MMU).
> > > >> >
> > > >> > A good example of such a device would be SATA AHCI controller given
> > > >> > to a
> > > >> > Guest/VM as direct access (using SystemMMU) and Guest
> > > >> > bootloader/firmware
> > > >> > accessing this SATA AHCI controller to load kernel images from SATA
> > > >> > disk.
> > > >> > In this situation, we will have to hack Guest bootloader/firmware
> > > >> > AHCI driver to
> > > >> > explicitly flushes to Guest RAM (because have HCR_EL2.DC=1).
> > > >>
> > > >> OK. So *that* looks like a valid argument against HCR_EL2.DC==1. Out
> > of
> > > >> curiosity: is that a made up example or something you actually have?
> > > >>
> > > >> Back to square one:
> > > >> Can you please benchmark the various cache cleaning options (global at
> > > >> startup time, per-page on S2 translation fault, and user-space)?
> > > >>
> > > > Eh, why is this a more valid argument than the vgic?  The device
> > > > passthrough Stage-2 mappings would still have the Stage-2 memory
> > > > attributes to configure that memory region as device memory.  Why is it
> > > > relevant if the device is DMA-capable in this context?
> > > >
> > >
> > > Most ARM bootloader/firmware run with MMU off hence, they will not do
> > > explicit cache flushes when programming DMA-capable device. Now If
> > > HCR_EL2.DC=1 then Guest RAM updates done by bootloader/firmware
> > > will not be visible to DMA-capable device.
> > >
> > Read my question again: The bootloaders running with the MMU off in a VM
> > will only disable the MMU for Stage-1 translations.  Stage-2
> > translations are still enforced using hte Stage-2 page tables and their
> > attributes for all mappings to devices will still enforce
> > strongly-ordered device type memory.
> >
> 
> Please read my reply again. Also try to read-up SATA AHCI spec.
> 
>

Can you please explain how the specs realate to my question?

The only case I can see is if the guest puts data in a DMA region that
it expects the device to read.  Is this the case?

Just so we're clear, this is quite different from programming the device
through the device memory.

-Christoffer