From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55403) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1erNeX-0005Lo-1R for qemu-devel@nongnu.org; Thu, 01 Mar 2018 07:49:48 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1erNeP-0000Ui-Hm for qemu-devel@nongnu.org; Thu, 01 Mar 2018 07:49:41 -0500 Date: Thu, 1 Mar 2018 12:49:11 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20180301124911.GE2994@work-vm> References: <20180228195320.165230-1-borntraeger@de.ibm.com> <79f7059b-f2d3-a758-6bb9-29433b31b313@redhat.com> <20180301092442.GA2994@work-vm> <20180301114543.GC2994@work-vm> <69654fb2-f5ba-c23b-f6f5-1b559692cf37@de.ibm.com> <20180301122854.GD2994@work-vm> <416dbf71-c311-1f40-baa6-414cdf220f4d@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <416dbf71-c311-1f40-baa6-414cdf220f4d@de.ibm.com> Subject: Re: [Qemu-devel] [PATCH 1/1] s390/kvm: implement clearing part of IPL clear List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Christian Borntraeger Cc: Thomas Huth , qemu-s390x , qemu-devel , Cornelia Huck , David Hildenbrand , Halil Pasic , Janosch Frank , Paolo Bonzini * Christian Borntraeger (borntraeger@de.ibm.com) wrote: > > > On 03/01/2018 01:28 PM, Dr. David Alan Gilbert wrote: > > * Christian Borntraeger (borntraeger@de.ibm.com) wrote: > >> > >> > >> On 03/01/2018 12:45 PM, Dr. David Alan Gilbert wrote: > >>> * Christian Borntraeger (borntraeger@de.ibm.com) wrote: > >>>> > >>>> > >>>> On 03/01/2018 10:24 AM, Dr. David Alan Gilbert wrote: > >>>>> * Thomas Huth (thuth@redhat.com) wrote: > >>>>>> On 28.02.2018 20:53, Christian Borntraeger wrote: > >>>>>>> When a guests reboots with diagnose 308 subcode 3 it requests the memory > >>>>>>> to be cleared. We did not do it so far. This does not only violate the > >>>>>>> architecture, it also misses the chance to free up that memory on > >>>>>>> reboot, which would help on host memory over commitment. By using > >>>>>>> ram_block_discard_range we can cover both cases. > >>>>>> > >>>>>> Sounds like a good idea. I wonder whether that release_all_ram() > >>>>>> function should maybe rather reside in exec.c, so that other machines > >>>>>> that want to clear all RAM at reset time can use it, too? > >>>>>> > >>>>>>> Signed-off-by: Christian Borntraeger > >>>>>>> --- > >>>>>>> target/s390x/kvm.c | 19 +++++++++++++++++++ > >>>>>>> 1 file changed, 19 insertions(+) > >>>>>>> > >>>>>>> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c > >>>>>>> index 8f3a422288..2e145ad5c3 100644 > >>>>>>> --- a/target/s390x/kvm.c > >>>>>>> +++ b/target/s390x/kvm.c > >>>>>>> @@ -34,6 +34,8 @@ > >>>>>>> #include "qapi/error.h" > >>>>>>> #include "qemu/error-report.h" > >>>>>>> #include "qemu/timer.h" > >>>>>>> +#include "qemu/rcu_queue.h" > >>>>>>> +#include "sysemu/cpus.h" > >>>>>>> #include "sysemu/sysemu.h" > >>>>>>> #include "sysemu/hw_accel.h" > >>>>>>> #include "hw/boards.h" > >>>>>>> @@ -41,6 +43,7 @@ > >>>>>>> #include "sysemu/device_tree.h" > >>>>>>> #include "exec/gdbstub.h" > >>>>>>> #include "exec/address-spaces.h" > >>>>>>> +#include "exec/ram_addr.h" > >>>>>>> #include "trace.h" > >>>>>>> #include "qapi-event.h" > >>>>>>> #include "hw/s390x/s390-pci-inst.h" > >>>>>>> @@ -1841,6 +1844,14 @@ static int kvm_arch_handle_debug_exit(S390CPU *cpu) > >>>>>>> return ret; > >>>>>>> } > >>>>>>> > >>>>>>> +static void release_all_rams(void) > >>>>>> > >>>>>> s/rams/ram/ maybe? > >>>>>> > >>>>>>> +{ > >>>>>>> + struct RAMBlock *rb; > >>>>>>> + > >>>>>>> + QLIST_FOREACH_RCU(rb, &ram_list.blocks, next) > >>>>>>> + ram_block_discard_range(rb, 0, rb->used_length); > >>>>>> > >>>>>> From a coding style point of view, I think there should be curly braces > >>>>>> around ram_block_discard_range() ? > >>>>> > >>>>> I think this might break if it happens during a postcopy migrate. > >>>>> The destination CPU is running, so it can do a reboot at just the wrong > >>>>> time; and then the pages (that are protected by userfaultfd) would get > >>>>> deallocated and trigger userfaultfd requests if accessed. > >>>> > >>>> Yes, userfaultd/postcopy is really fragile and relies on things that are not > >>>> necessarily true (e.g. virito-balloon can also invalidate pages). > >>> > >>> That's why we use qemu_balloon_inhibit around postcopy to stop > >>> ballooning; I'm not aware of anything else that does the same. > >> > >> we also have at least the pte_unused thing in mm/rmap.c that clearly > >> predates userfaultfd. We might need to look into this as well.... > > > > I've not come across that; what does that do? > > It can drop a page on page out if the page is no longer of value. It is used by > the CMMA (guest page hinting) code of s390x. > > see kernel mm/rmap.c > > > static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, > unsigned long address, void *arg) > { > [...] > } else if (pte_unused(pteval)) { > /* > * The guest indicated that the page content is of no > * interest anymore. Simply discard the pte, vmscan > * will take care of the rest. > */ > dec_mm_counter(mm, mm_counter(page)); > /* We have to invalidate as we cleared the pte */ > mmu_notifier_invalidate_range(mm, address, > address + PAGE_SIZE); > } else if (IS_ENABLED(CONFIG_MIGRATION) && > (flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))) { > [...] OK, probably best to check with Andrea what the best way to get that happy with userfault is. > > > >>> > >>>> The right thing here would be to actually terminate the postcopy migrate but > >>>> return it as "successful" (since we are going to clear that RAM anyway). Do > >>>> you see a good way to achieve that? > >>> > >>> There's no current mechanism to do it; I think it would have to involve > >>> some interaction with the source as well though to tell it that you > >>> didn't need that area of RAM anyway. > >>> > >>> However, there are more problems: > >>> a) Even forgetting the userfault problem, this is racy since during > >>> postcopy you're still receiving blocks from the source at the same time; > >>> so some of the area that you've discarded might get overwritten by data > >>> from the source. > >> > >> So how do you handle the case when the target system writes to memory > >> that is still in flight? Can we build on that mechanism? > > > > Once we've entered postcopy, a page is basically in one of two states: > > a) Not yet received - i.e. marked absent with MADV_DONTNEED; if the > > guest tries to write to it then it'll block with userfault and ask the > > source for the page; so the write wont happen until the page arrives. > > b) Received - we've already got the page from the source; the source > > never resends a page (once in postcopy) so now the destination can just > > write to the page. > > > > Once in postcopy, a page is received at most once (i.e. if it's not > > been received during precopy). > > > > I can imagine two ways of curing it: > > a) Simple but slow; just read all the pages before doing the > > discard, this forces it to wait for the pages to be received. > > b) More complex but fast; Add a message on the return path to the > > source telling it that you're going to discard a range; the source then > > marks it's notes as cleared for those pages and then sends some form of > > ack, and at that point you drop it. > > this looks like the most promising approach, but some work. Yes, you can add a new MIG_RP_MSG_ type for the destination to tell the source; that's pretty easy. Remember that there will still be pages in flight after you've sent this message so you'll have to wait for those to clear out. > > > > A 3rd; incomplete way; would be just to drop the userfaultfd on the > > destination for the RAMBlocks that are being cleared; but this does > > leave the source state in a bit of a mess. > > > > > >>> b) Your release_all_rams seems to do all RAM Blocks - won't that nuke > >>> any ROMs as well? Or maybe even flash? > >> > >> ROMs loaded with load_elf (like our s390-ccw.img) are reloaded on every reset. > >> See rom_reset in /hw/core/loader.c > > > > Ah, so this is happening after your reset code you've added? > > Yes, I am stopping all CPU, clear the memory. And then I call system_reset. OK. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK