From: Sean Christopherson <seanjc@google.com>
To: Isaku Yamahata <isaku.yamahata@intel.com>
Cc: David Matlack <dmatlack@google.com>,
kvm@vger.kernel.org, isaku.yamahata@gmail.com,
linux-kernel@vger.kernel.org,
Paolo Bonzini <pbonzini@redhat.com>,
Michael Roth <michael.roth@amd.com>,
Federico Parola <federico.parola@polito.it>,
isaku.yamahata@linux.intel.com
Subject: Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API
Date: Wed, 3 Apr 2024 15:42:47 -0700 [thread overview]
Message-ID: <Zg3bZ3MQaBvC5LML@google.com> (raw)
In-Reply-To: <20240403220023.GL2444378@ls.amr.corp.intel.com>
On Wed, Apr 03, 2024, Isaku Yamahata wrote:
> On Wed, Apr 03, 2024 at 11:30:21AM -0700,
> Sean Christopherson <seanjc@google.com> wrote:
>
> > On Tue, Mar 19, 2024, Isaku Yamahata wrote:
> > > On Wed, Mar 06, 2024 at 06:09:54PM -0800,
> > > Isaku Yamahata <isaku.yamahata@linux.intel.com> wrote:
> > >
> > > > On Wed, Mar 06, 2024 at 04:53:41PM -0800,
> > > > David Matlack <dmatlack@google.com> wrote:
> > > >
> > > > > On 2024-03-01 09:28 AM, isaku.yamahata@intel.com wrote:
> > > > > > From: Isaku Yamahata <isaku.yamahata@intel.com>
> > > > > >
> > > > > > Implementation:
> > > > > > - x86 KVM MMU
> > > > > > In x86 KVM MMU, I chose to use kvm_mmu_do_page_fault(). It's not confined to
> > > > > > KVM TDP MMU. We can restrict it to KVM TDP MMU and introduce an optimized
> > > > > > version.
> > > > >
> > > > > Restricting to TDP MMU seems like a good idea. But I'm not quite sure
> > > > > how to reliably do that from a vCPU context. Checking for TDP being
> > > > > enabled is easy, but what if the vCPU is in guest-mode?
> > > >
> > > > As you pointed out in other mail, legacy KVM MMU support or guest-mode will be
> > > > troublesome.
> >
> > Why is shadow paging troublesome? I don't see any obvious issues with effectively
> > prefetching into a shadow MMU with read fault semantics. It might be pointless
> > and wasteful, as the guest PTEs need to be in place, but that's userspace's problem.
>
> The populating address for shadow paging is GVA, not GPA. I'm not sure if
> that's what the user space wants. If it's user-space problem, I'm fine.
/facepalm
> > Pre-populating is the primary use case, but that could happen if L2 is active,
> > e.g. after live migration.
> >
> > I'm not necessarily opposed to initially adding support only for the TDP MMU, but
> > if the delta to also support the shadow MMU is relatively small, my preference
> > would be to add the support right away. E.g. to give us confidence that the uAPI
> > can work for multiple MMUs, and so that we don't have to write documentation for
> > x86 to explain exactly when it's legal to use the ioctl().
>
> If we call kvm_mmu.page_fault() without caring of what address will be
> populated, I don't see the big difference.
Ignore me, I completely spaced that shadow MMUs don't operate on an L1 GPA. I
100% agree that restricting this to TDP, at least for the initial merge, is the
way to go. A uAPI where the type of address varies based on the vCPU mode and
MMU type would be super ugly, and probably hard to use.
At that point, I don't have a strong preference as to whether or not direct
legacy/shadow MMUs are supported. That said, I think it can (probably should?)
be done in a way where it more or less Just Works, e.g. by having a function hook
in "struct kvm_mmu".
next prev parent reply other threads:[~2024-04-03 22:42 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-01 17:28 [RFC PATCH 0/8] KVM: Prepopulate guest memory API isaku.yamahata
2024-03-01 17:28 ` [RFC PATCH 1/8] KVM: Document KVM_MAP_MEMORY ioctl isaku.yamahata
2024-03-07 0:43 ` David Matlack
2024-03-07 1:29 ` Isaku Yamahata
2024-03-07 12:30 ` Huang, Kai
2024-03-07 20:33 ` Isaku Yamahata
2024-03-08 0:20 ` Huang, Kai
2024-03-08 0:56 ` David Matlack
2024-03-08 1:28 ` Sean Christopherson
2024-03-08 2:19 ` Isaku Yamahata
2024-03-10 23:12 ` Michael Roth
2024-03-11 1:05 ` Huang, Kai
2024-03-11 1:08 ` Huang, Kai
2024-03-12 1:34 ` Isaku Yamahata
2024-03-01 17:28 ` [RFC PATCH 2/8] KVM: Add KVM_MAP_MEMORY vcpu ioctl to pre-populate guest memory isaku.yamahata
2024-03-07 0:49 ` David Matlack
2024-03-07 2:52 ` Isaku Yamahata
2024-03-07 12:45 ` Huang, Kai
2024-03-07 20:41 ` Isaku Yamahata
2024-03-11 17:23 ` Sean Christopherson
2024-03-11 22:19 ` Isaku Yamahata
2024-03-01 17:28 ` [RFC PATCH 3/8] KVM: x86/mmu: Introduce initialier macro for struct kvm_page_fault isaku.yamahata
2024-03-11 17:24 ` Sean Christopherson
2024-03-11 22:56 ` Isaku Yamahata
2024-03-01 17:28 ` [RFC PATCH 4/8] KVM: x86/mmu: Factor out kvm_mmu_do_page_fault() isaku.yamahata
2024-03-01 17:28 ` [RFC PATCH 5/8] KVM: x86/mmu: Introduce kvm_mmu_map_page() for prepopulating guest memory isaku.yamahata
2024-03-07 0:38 ` David Matlack
2024-03-19 15:53 ` Isaku Yamahata
2024-03-11 17:29 ` Sean Christopherson
2024-03-11 22:57 ` Isaku Yamahata
2024-03-01 17:28 ` [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map_memory() isaku.yamahata
2024-03-07 0:30 ` David Matlack
2024-03-07 0:36 ` David Matlack
2024-03-07 1:51 ` Isaku Yamahata
2024-03-19 16:26 ` Isaku Yamahata
2024-04-03 23:15 ` Sean Christopherson
2024-03-07 1:34 ` Isaku Yamahata
2024-03-11 23:26 ` Sean Christopherson
2024-03-12 12:38 ` Huang, Kai
2024-03-12 14:20 ` Sean Christopherson
2024-03-12 21:41 ` Huang, Kai
2024-03-12 21:46 ` Huang, Kai
2024-03-12 23:03 ` Sean Christopherson
2024-03-01 17:28 ` [RFC PATCH 7/8] KVM: x86: Add hooks in kvm_arch_vcpu_map_memory() isaku.yamahata
2024-03-01 17:28 ` [RFC PATCH 8/8] KVM: selftests: x86: Add test for KVM_MAP_MEMORY isaku.yamahata
2024-03-07 0:53 ` [RFC PATCH 0/8] KVM: Prepopulate guest memory API David Matlack
2024-03-07 2:09 ` Isaku Yamahata
2024-03-19 16:33 ` Isaku Yamahata
2024-04-03 18:30 ` Sean Christopherson
2024-04-03 22:00 ` Isaku Yamahata
2024-04-03 22:42 ` Sean Christopherson [this message]
2024-03-11 3:20 ` Michael Roth
2024-03-11 23:44 ` Sean Christopherson
2024-03-12 1:32 ` Isaku Yamahata
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