From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
Peter Xu <peterx@redhat.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Paul Mackerras" <paulus@ozlabs.org>,
"Christian Borntraeger" <borntraeger@de.ibm.com>,
"Janosch Frank" <frankja@linux.ibm.com>,
"David Hildenbrand" <david@redhat.com>,
"Cornelia Huck" <cohuck@redhat.com>,
"Wanpeng Li" <wanpengli@tencent.com>,
"Jim Mattson" <jmattson@google.com>,
"Joerg Roedel" <joro@8bytes.org>, "Marc Zyngier" <maz@kernel.org>,
"James Morse" <james.morse@arm.com>,
"Julien Thierry" <julien.thierry.kdev@gmail.com>,
"Suzuki K Poulose" <suzuki.poulose@arm.com>,
linux-mips@vger.kernel.org, kvm@vger.kernel.org,
kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
"Christoffer Dall" <christoffer.dall@arm.com>,
"Philippe Mathieu-Daudé" <f4bug@amsat.org>
Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions
Date: Mon, 17 Feb 2020 16:39:39 +0100 [thread overview]
Message-ID: <87sgj99q9w.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200208012938.GC15581@linux.intel.com>
Sean Christopherson <sean.j.christopherson@intel.com> writes:
> On Fri, Feb 07, 2020 at 07:53:34PM -0500, Peter Xu wrote:
>> On Fri, Feb 07, 2020 at 04:42:33PM -0800, Sean Christopherson wrote:
>> > On Fri, Feb 07, 2020 at 07:18:32PM -0500, Peter Xu wrote:
>> > > On Fri, Feb 07, 2020 at 11:45:32AM -0800, Sean Christopherson wrote:
>> > > > +Vitaly for HyperV
>> > > >
>> > > > On Thu, Feb 06, 2020 at 04:41:06PM -0500, Peter Xu wrote:
>> > > > > On Thu, Feb 06, 2020 at 01:21:20PM -0800, Sean Christopherson wrote:
>> > > > > > On Thu, Feb 06, 2020 at 03:02:00PM -0500, Peter Xu wrote:
>> > > > > > > But that matters to this patch because if MIPS can use
>> > > > > > > kvm_flush_remote_tlbs(), then we probably don't need this
>> > > > > > > arch-specific hook any more and we can directly call
>> > > > > > > kvm_flush_remote_tlbs() after sync dirty log when flush==true.
>> > > > > >
>> > > > > > Ya, the asid_flush_mask in kvm_vz_flush_shadow_all() is the only thing
>> > > > > > that prevents calling kvm_flush_remote_tlbs() directly, but I have no
>> > > > > > clue as to the important of that code.
>> > > > >
>> > > > > As said above I think the x86 lockdep is really not necessary, then
>> > > > > considering MIPS could be the only one that will use the new hook
>> > > > > introduced in this patch... Shall we figure that out first?
>> > > >
>> > > > So I prepped a follow-up patch to make kvm_arch_dirty_log_tlb_flush() a
>> > > > MIPS-only hook and use kvm_flush_remote_tlbs() directly for arm and x86,
>> > > > but then I realized x86 *has* a hook to do a precise remote TLB flush.
>> > > > There's even an existing kvm_flush_remote_tlbs_with_address() call on a
>> > > > memslot, i.e. this exact scenario. So arguably, x86 should be using the
>> > > > more precise flush and should keep kvm_arch_dirty_log_tlb_flush().
>> > > >
>> > > > But, the hook is only used when KVM is running as an L1 on top of HyperV,
>> > > > and I assume dirty logging isn't used much, if at all, for L1 KVM on
>> > > > HyperV?
>> > > >
>> > > > I see three options:
>> > > >
>> > > > 1. Make kvm_arch_dirty_log_tlb_flush() MIPS-only and call
>> > > > kvm_flush_remote_tlbs() directly for arm and x86. Add comments to
>> > > > explain when an arch should implement kvm_arch_dirty_log_tlb_flush().
>> > > >
>> > > > 2. Change x86 to use kvm_flush_remote_tlbs_with_address() when flushing
>> > > > a memslot after the dirty log is grabbed by userspace.
>> > > >
>> > > > 3. Keep the resulting code as is, but add a comment in x86's
>> > > > kvm_arch_dirty_log_tlb_flush() to explain why it uses
>> > > > kvm_flush_remote_tlbs() instead of the with_address() variant.
>> > > >
>> > > > I strongly prefer to (2) or (3), but I'll defer to Vitaly as to which of
>> > > > those is preferable.
>> > > >
>> > > > I don't like (1) because (a) it requires more lines code (well comments),
>> > > > to explain why kvm_flush_remote_tlbs() is the default, and (b) it would
>> > > > require even more comments, which would be x86-specific in generic KVM,
>> > > > to explain why x86 doesn't use its with_address() flush, or we'd lost that
>> > > > info altogether.
>> > > >
>> > >
>> > > I proposed the 4th solution here:
>> > >
>> > > https://lore.kernel.org/kvm/20200207223520.735523-1-peterx@redhat.com/
>> > >
>> > > I'm not sure whether that's acceptable, but if it can, then we can
>> > > drop the kvm_arch_dirty_log_tlb_flush() hook, or even move on to
>> > > per-slot tlb flushing.
>> >
>> > This effectively is per-slot TLB flushing, it just has a different name.
>> > I.e. s/kvm_arch_dirty_log_tlb_flush/kvm_arch_flush_remote_tlbs_memslot.
>> > I'm not opposed to that name change. And on second and third glance, I
>> > probably prefer it. That would more or less follow the naming of
>> > kvm_arch_flush_shadow_all() and kvm_arch_flush_shadow_memslot().
>>
>> Note that the major point of the above patchset is not about doing tlb
>> flush per-memslot or globally. It's more about whether we can provide
>> a common entrance for TLB flushing. Say, after that series, we should
>> be able to flush TLB on all archs (majorly, including MIPS) as:
>>
>> kvm_flush_remote_tlbs(kvm);
>>
>> And with the same idea we can also introduce the ranged version.
>>
>> >
>> > I don't want to go straight to kvm_arch_flush_remote_tlb_with_address()
>> > because that loses the important distinction (on x86) that slots_lock is
>> > expected to be held.
>>
>> Sorry I'm still puzzled on why that lockdep is so important and
>> special for x86... For example, what if we move that lockdep to the
>> callers of the kvm_arch_dirty_log_tlb_flush() calls so it protects
>> even more arch (where we do get/clear dirty log)? IMHO the callers
>> must be with the slots_lock held anyways no matter for x86 or not.
>
>
> Following the breadcrumbs leads to the comment in
> kvm_mmu_slot_remove_write_access(), which says:
>
> /*
> * kvm_mmu_slot_remove_write_access() and kvm_vm_ioctl_get_dirty_log()
> * which do tlb flush out of mmu-lock should be serialized by
> * kvm->slots_lock otherwise tlb flush would be missed.
> */
>
> I.e. write-protecting a memslot and grabbing the dirty log for the memslot
> need to be serialized. It's quite obvious *now* that get_dirty_log() holds
> slots_lock, but the purpose of lockdep assertions isn't just to verify the
> current functionality, it's to help ensure the correctness for future code
> and to document assumptions in the code.
>
> Digging deeper, there are four functions, all related to dirty logging, in
> the x86 mmu that basically open code what x86's
> kvm_arch_flush_remote_tlbs_memslot() would look like if it uses the range
> based flushing.
>
> Unless it's functionally incorrect (Vitaly?), going with option (2) and
> naming the hook kvm_arch_flush_remote_tlbs_memslot() seems like the obvious
> choice, e.g. the final cleanup gives this diff stat:
(I apologize again for not replying in time)
I think this is a valid approach and your option (2) would also be my
choice. I also don't think there's going to be a problem when (if)
Hyper-V adds support for PML (eVMCSv2?).
>
> arch/x86/kvm/mmu/mmu.c | 34 +++++++++-------------------------
> 1 file changed, 9 insertions(+), 25 deletions(-)
>
Looks nice :-)
--
Vitaly
next prev parent reply other threads:[~2020-02-17 15:39 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-21 22:31 [PATCH v5 00/19] KVM: Dynamically size memslot arrays Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 01/19] KVM: x86: Allocate new rmap and large page tracking when moving memslot Sean Christopherson
2020-02-05 21:49 ` Peter Xu
2020-02-05 23:55 ` Sean Christopherson
2020-02-06 2:00 ` Peter Xu
2020-02-06 2:17 ` Sean Christopherson
2020-02-06 2:58 ` Peter Xu
2020-02-06 5:05 ` Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 02/19] KVM: Reinstall old memslots if arch preparation fails Sean Christopherson
2020-02-05 22:08 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 03/19] KVM: Don't free new memslot if allocation of said memslot fails Sean Christopherson
2020-02-05 22:28 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 04/19] KVM: PPC: Move memslot memory allocation into prepare_memory_region() Sean Christopherson
2020-02-05 22:41 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 05/19] KVM: x86: Allocate memslot resources during prepare_memory_region() Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 06/19] KVM: Drop kvm_arch_create_memslot() Sean Christopherson
2020-02-05 22:45 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 07/19] KVM: Explicitly free allocated-but-unused dirty bitmap Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 08/19] KVM: Refactor error handling for setting memory region Sean Christopherson
2020-02-05 22:48 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 09/19] KVM: Move setting of memslot into helper routine Sean Christopherson
2020-02-06 16:26 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 10/19] KVM: Drop "const" attribute from old memslot in commit_memory_region() Sean Christopherson
2020-02-06 16:26 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 11/19] KVM: x86: Free arrays for old memslot when moving memslot's base gfn Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 12/19] KVM: Move memslot deletion to helper function Sean Christopherson
2020-02-06 16:14 ` Peter Xu
2020-02-06 16:28 ` Sean Christopherson
2020-02-06 16:51 ` Peter Xu
2020-02-07 17:59 ` Sean Christopherson
2020-02-07 18:07 ` Sean Christopherson
2020-02-07 18:17 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 13/19] KVM: Simplify kvm_free_memslot() and all its descendents Sean Christopherson
2020-02-06 16:29 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 14/19] KVM: Clean up local variable usage in __kvm_set_memory_region() Sean Christopherson
2020-02-06 19:06 ` Peter Xu
2020-02-06 19:22 ` Sean Christopherson
2020-02-06 19:36 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Sean Christopherson
2020-02-06 20:02 ` Peter Xu
2020-02-06 21:21 ` Sean Christopherson
2020-02-06 21:41 ` Peter Xu
2020-02-07 19:45 ` Sean Christopherson
2020-02-08 0:18 ` Peter Xu
2020-02-08 0:42 ` Sean Christopherson
2020-02-08 0:53 ` Peter Xu
2020-02-08 1:29 ` Sean Christopherson
2020-02-17 15:39 ` Vitaly Kuznetsov [this message]
2020-02-18 17:10 ` Sean Christopherson
2020-02-17 15:35 ` Vitaly Kuznetsov
2020-02-06 21:24 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log() Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 17/19] KVM: Terminate memslot walks via used_slots Sean Christopherson
2020-02-06 21:09 ` Peter Xu
2020-02-07 18:33 ` Sean Christopherson
2020-02-07 20:39 ` Peter Xu
2020-02-07 21:10 ` Sean Christopherson
2020-02-07 21:46 ` Peter Xu
2020-02-07 22:03 ` Sean Christopherson
2020-02-07 22:24 ` Peter Xu
2020-01-21 22:31 ` [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots Sean Christopherson
2020-02-06 22:12 ` Peter Xu
2020-02-07 15:38 ` Sean Christopherson
2020-02-07 16:05 ` Peter Xu
2020-02-07 16:15 ` Sean Christopherson
2020-02-07 16:37 ` Peter Xu
2020-02-07 16:47 ` Sean Christopherson
2020-01-21 22:31 ` [PATCH v5 19/19] KVM: selftests: Add test for KVM_SET_USER_MEMORY_REGION Sean Christopherson
2020-02-06 22:30 ` Peter Xu
2020-02-06 23:11 ` Sean Christopherson
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