On 24.07.2013, at 11:11, Bhushan Bharat-R65777 wrote: > > >> -----Original Message----- >> From: Alexander Graf [mailto:agraf@xxxxxxx] >> Sent: Wednesday, July 24, 2013 1:55 PM >> To: "“tiejun.chen”" >> Cc: Bhushan Bharat-R65777; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx list; >> Wood Scott-B07421; Gleb Natapov; Paolo Bonzini >> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency only for kernel >> managed pages >> >> >> On 24.07.2013, at 04:26, “tiejun.chen” wrote: >> >>> On 07/18/2013 06:27 PM, Alexander Graf wrote: >>>> >>>> On 18.07.2013, at 12:19, “tiejun.chen” wrote: >>>> >>>>> On 07/18/2013 06:12 PM, Alexander Graf wrote: >>>>>> >>>>>> On 18.07.2013, at 12:08, “tiejun.chen” wrote: >>>>>> >>>>>>> On 07/18/2013 05:48 PM, Alexander Graf wrote: >>>>>>>> >>>>>>>> On 18.07.2013, at 10:25, Bhushan Bharat-R65777 wrote: >>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> -----Original Message----- >>>>>>>>>> From: Bhushan Bharat-R65777 >>>>>>>>>> Sent: Thursday, July 18, 2013 1:53 PM >>>>>>>>>> To: '" tiejun.chen "' >>>>>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; >>>>>>>>>> agraf@xxxxxxx; Wood Scott- >>>>>>>>>> B07421 >>>>>>>>>> Subject: RE: [PATCH 2/2] kvm: powerpc: set cache coherency only >>>>>>>>>> for kernel managed pages >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>> -----Original Message----- >>>>>>>>>>> From: " tiejun.chen " [mailto:tiejun.chen@xxxxxxxxxxxxx] >>>>>>>>>>> Sent: Thursday, July 18, 2013 1:52 PM >>>>>>>>>>> To: Bhushan Bharat-R65777 >>>>>>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; >>>>>>>>>>> agraf@xxxxxxx; Wood >>>>>>>>>>> Scott- >>>>>>>>>>> B07421 >>>>>>>>>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency >>>>>>>>>>> only for kernel managed pages >>>>>>>>>>> >>>>>>>>>>> On 07/18/2013 04:08 PM, Bhushan Bharat-R65777 wrote: >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> -----Original Message----- >>>>>>>>>>>>> From: kvm-ppc-owner@xxxxxxxxxxxxxxx >>>>>>>>>>>>> [mailto:kvm-ppc-owner@xxxxxxxxxxxxxxx] On Behalf Of " tiejun.chen " >>>>>>>>>>>>> Sent: Thursday, July 18, 2013 1:01 PM >>>>>>>>>>>>> To: Bhushan Bharat-R65777 >>>>>>>>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; >>>>>>>>>>>>> agraf@xxxxxxx; Wood >>>>>>>>>>>>> Scott- >>>>>>>>>>>>> B07421 >>>>>>>>>>>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency >>>>>>>>>>>>> only for kernel managed pages >>>>>>>>>>>>> >>>>>>>>>>>>> On 07/18/2013 03:12 PM, Bhushan Bharat-R65777 wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>>> -----Original Message----- >>>>>>>>>>>>>>> From: " tiejun.chen " [mailto:tiejun.chen@xxxxxxxxxxxxx] >>>>>>>>>>>>>>> Sent: Thursday, July 18, 2013 11:56 AM >>>>>>>>>>>>>>> To: Bhushan Bharat-R65777 >>>>>>>>>>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; >>>>>>>>>>>>>>> agraf@xxxxxxx; Wood >>>>>>>>>>>>>>> Scott- B07421; Bhushan Bharat-R65777 >>>>>>>>>>>>>>> Subject: Re: [PATCH 2/2] kvm: powerpc: set cache coherency >>>>>>>>>>>>>>> only for kernel managed pages >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On 07/18/2013 02:04 PM, Bharat Bhushan wrote: >>>>>>>>>>>>>>>> If there is a struct page for the requested mapping then >>>>>>>>>>>>>>>> it's normal DDR and the mapping sets "M" bit (coherent, >>>>>>>>>>>>>>>> cacheable) else this is treated as I/O and we set "I + >>>>>>>>>>>>>>>> G" (cache inhibited, >>>>>>>>>>>>>>>> guarded) >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> This helps setting proper TLB mapping for direct assigned >>>>>>>>>>>>>>>> device >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Signed-off-by: Bharat Bhushan >>>>>>>>>>>>>>>> <bharat.bhushan@xxxxxxxxxxxxx> >>>>>>>>>>>>>>>> --- >>>>>>>>>>>>>>>> arch/powerpc/kvm/e500_mmu_host.c | 17 ++++++++++++----- >>>>>>>>>>>>>>>> 1 files changed, 12 insertions(+), 5 deletions(-) >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> diff --git a/arch/powerpc/kvm/e500_mmu_host.c >>>>>>>>>>>>>>>> b/arch/powerpc/kvm/e500_mmu_host.c >>>>>>>>>>>>>>>> index 1c6a9d7..089c227 100644 >>>>>>>>>>>>>>>> --- a/arch/powerpc/kvm/e500_mmu_host.c >>>>>>>>>>>>>>>> +++ b/arch/powerpc/kvm/e500_mmu_host.c >>>>>>>>>>>>>>>> @@ -64,13 +64,20 @@ static inline u32 >>>>>>>>>>>>>>>> e500_shadow_mas3_attrib(u32 mas3, int >>>>>>>>>>>>>>> usermode) >>>>>>>>>>>>>>>> return mas3; >>>>>>>>>>>>>>>> } >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> -static inline u32 e500_shadow_mas2_attrib(u32 mas2, int >>>>>>>>>>>>>>>> usermode) >>>>>>>>>>>>>>>> +static inline u32 e500_shadow_mas2_attrib(u32 mas2, >>>>>>>>>>>>>>>> +pfn_t pfn) >>>>>>>>>>>>>>>> { >>>>>>>>>>>>>>>> + u32 mas2_attr; >>>>>>>>>>>>>>>> + >>>>>>>>>>>>>>>> + mas2_attr = mas2 & MAS2_ATTRIB_MASK; >>>>>>>>>>>>>>>> + >>>>>>>>>>>>>>>> + if (!pfn_valid(pfn)) { >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Why not directly use kvm_is_mmio_pfn()? >>>>>>>>>>>>>> >>>>>>>>>>>>>> What I understand from this function (someone can correct >>>>>>>>>>>>>> me) is that it >>>>>>>>>>>>> returns "false" when the page is managed by kernel and is >>>>>>>>>>>>> not marked as RESERVED (for some reason). For us it does not >>>>>>>>>>>>> matter whether the page is reserved or not, if it is kernel >>>>>>>>>>>>> visible page then it >>>>>>>>>> is DDR. >>>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> I think you are setting I|G by addressing all mmio pages, >>>>>>>>>>>>> right? If so, >>>>>>>>>>>>> >>>>>>>>>>>>> KVM: direct mmio pfn check >>>>>>>>>>>>> >>>>>>>>>>>>> Userspace may specify memory slots that are backed by >>>>>>>>>>>>> mmio pages rather than >>>>>>>>>>>>> normal RAM. In some cases it is not enough to identify >>>>>>>>>>>>> these mmio >>>>>>>>>>> pages >>>>>>>>>>>>> by pfn_valid(). This patch adds checking the PageReserved as >> well. >>>>>>>>>>>> >>>>>>>>>>>> Do you know what are those "some cases" and how checking >>>>>>>>>>>> PageReserved helps in >>>>>>>>>>> those cases? >>>>>>>>>>> >>>>>>>>>>> No, myself didn't see these actual cases in qemu,too. But this >>>>>>>>>>> should be chronically persistent as I understand ;-) >>>>>>>>>> >>>>>>>>>> Then I will wait till someone educate me :) >>>>>>>>> >>>>>>>>> The reason is , kvm_is_mmio_pfn() function looks pretty heavy and I do >> not want to call this for all tlbwe operation unless it is necessary. >>>>>>>> >>>>>>>> It certainly does more than we need and potentially slows down the fast >> path (RAM mapping). The only thing it does on top of "if (pfn_valid())" is to >> check for pages that are declared reserved on the host. This happens in 2 cases: >>>>>>>> >>>>>>>> 1) Non cache coherent DMA >>>>>>>> 2) Memory hot remove >>>>>>>> >>>>>>>> The non coherent DMA case would be interesting, as with the mechanism as >> it is in place in Linux today, we could potentially break normal guest operation >> if we don't take it into account. However, it's Kconfig guarded by: >>>>>>>> >>>>>>>> depends on 4xx || 8xx || E200 || PPC_MPC512x || GAMECUBE_COMMON >>>>>>>> default n if PPC_47x >>>>>>>> default y >>>>>>>> >>>>>>>> so we never hit it with any core we care about ;). >>>>>>>> >>>>>>>> Memory hot remove does not exist on e500 FWIW, so we don't have to worry >> about that one either. >>>>>>> >>>>>>> Thanks for this good information :) >>>>>>> >>>>>>> So why not limit those codes with CONFIG_MEMORY_HOTPLUG inside >> kvm_is_mmio_pfn() to make sure that check is only valid when that is really >> needed? This can decrease those unnecessary performance loss. >>>>>>> >>>>>>> If I'm wrong please correct me :) >>>>>> >>>>>> You're perfectly right, but this is generic KVM code. So it gets run across >> all architectures. What if someone has the great idea to add a new case here for >> x86, but doesn't tell us? In that case we potentially break x86. >>>>>> >>>>>> I'd rather not like to break x86 :). >>>>>> >>>>>> However, it'd be very interesting to see a benchmark with this change. Do >> you think you could just rip out the whole reserved check and run a few >> benchmarks and show us the results? >>>>>> >>>>> >>>>> Often what case should be adopted to validate this scenario? >>>> >>>> Something which hammers the TLB emulation heavily. I usually just run >>>> /bin/echo a thousand times in "time" and see how long it takes ;) >>>> >>> >>> I tried to run five times with this combination, "time `for ((i=0; i<5000; >> i++)); do /bin/echo; done`", to calculate the average value with this change: >>> >>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index >>> 1580dd4..5e8635b 100644 >>> --- a/virt/kvm/kvm_main.c >>> +++ b/virt/kvm/kvm_main.c >>> @@ -102,6 +102,10 @@ static bool largepages_enabled = true; >>> >>> bool kvm_is_mmio_pfn(pfn_t pfn) >>> { >>> +#ifdef CONFIG_MEMORY_HOTPLUG >> >> I'd feel safer if we narrow this down to e500. >> >>> + /* >>> + * Currently only in memory hot remove case we may still need this. >>> + */ >>> if (pfn_valid(pfn)) { >> >> We still have to check for pfn_valid, no? So the #ifdef should be down here. >> >>> int reserved; >>> struct page *tail = pfn_to_page(pfn); @@ -124,6 +128,7 >>> @@ bool kvm_is_mmio_pfn(pfn_t pfn) >>> } >>> return PageReserved(tail); >>> } >>> +#endif >>> >>> return true; >>> } >>> >>> Before apply this change: >>> >>> real (1m19.954s + 1m20.918s + 1m22.740s + 1m21.146s + 1m22.120s)/5= >> 1m21.376s >>> user (0m23.181s + 0m23.550s + 0m23.506s + 0m23.410s + 0m23.520s)/5= >> 0m23.433s >>> sys (0m49.087s + 0m49.563s + 0m51.758s + 0m50.290s + 0m51.047s)/5= 0m50.349s >>> >>> After apply this change: >>> >>> real (1m19.507s + 1m20.919s + 1m21.436s + 1m21.179s + 1m20.293s)/5= >> 1m20.667s >>> user (0m22.595s + 0m22.719s + 0m22.484s + 0m22.811s + 0m22.467s)/5= >> 0m22.615s >>> sys (0m48.841s + 0m49.929s + 0m50.310s + 0m49.813s + 0m48.587s)/5= 0m49.496s >>> >>> So, >>> >>> real (1m20.667s - 1m21.376s)/1m21.376s x 100% = -0.6% >>> user (0m22.615s - 0m23.433s)/0m23.433s x 100% = -3.5% >>> sys (0m49.496s - 0m50.349s)/0m50.349s x 100% = -1.7% >> >> Very nice, so there is a real world performance benefit to doing this. Then yes, >> I think it would make sense to change the global helper function to be fast on >> e500 and use that one from e500_shadow_mas2_attrib() instead. > > Are not we going to use page_is_ram() from e500_shadow_mas2_attrib() as Scott commented? rWhy aren't we using page_is_ram() in kvm_is_mmio_pfn()? Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
