From: Sean Christopherson <seanjc@google.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Paul Durrant <paul@xen.org>, Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 05/15] KVM: pfncache: remove KVM_GUEST_USES_PFN usage
Date: Mon, 27 Nov 2023 15:36:07 -0800 [thread overview]
Message-ID: <ZWUn50A5vxiTd-ZT@google.com> (raw)
In-Reply-To: <a1ebd80f87229fe513f9c2256982ef6c1d0cca2a.camel@infradead.org>
On Tue, Nov 21, 2023, David Woodhouse wrote:
> On Tue, 2023-11-21 at 18:02 +0000, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >
> > As noted in [1] the KVM_GUEST_USES_PFN usage flag is never set by any
> > callers of kvm_gpc_init(), which also makes the 'vcpu' argument redundant.
> > Moreover, all existing callers specify KVM_HOST_USES_PFN so the usage
> > check in hva_to_pfn_retry() and hence the 'usage' argument to
> > kvm_gpc_init() are also redundant.
> > Remove the pfn_cache_usage enumeration and remove the redundant arguments,
> > fields of struct gfn_to_hva_cache, and all the related code.
> >
> > [1] https://lore.kernel.org/all/ZQiR8IpqOZrOpzHC@google.com/
> >
> > Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>
> I think it's https://lore.kernel.org/all/ZBEEQtmtNPaEqU1i@google.com/
Yeah, that's the more important link.
> which is the key reference. I'm not sure I'm 100% on board, but I never
> got round to replying to Sean's email because it was one of those "put
> up or shut up situations" and I didn't have the bandwidth to actually
> write the code to prove my point.
>
> I think it *is* important to support non-pinned pages. There's a reason
> we even made the vapic page migratable. We want to support memory
> hotplug, we want to cope with machine checks telling us to move certain
> pages (which I suppose is memory hotplug). See commit 38b9917350cb
> ("kvm: vmx: Implement set_apic_access_page_addr") for example.
The vAPIC page is slightly different in that it effectively never opened a window
for page migration, i.e. once a vCPU was created that page was stuck. For nested
virtualization pages, the probability of being able to migrate a page at any given
time might be relatively low, but it's extremely unlikely for a page to be pinned
for the entire lifetime of a (L1) VM.
> I agree that in the first round of the nVMX code there were bugs. And
> sure, of *course* it isn't sufficient to wire up the invalidation
> without either a KVM_REQ_SOMETHIMG to put it back, or just a *check* on
> the corresponding gpc on the way back into the guest. We'd have worked
> that out.
Maybe. I spent most of a day, maybe longer, hacking at the nVMX code and was
unable to get line of sight to an end result that I felt would be worth pursuing.
I'm definitely not saying it's impossible, and I'm not dead set against
re-introducing KVM_GUEST_USES_PFN or similar, but a complete solution crosses the
threshold where it's unreasonable to ask/expect someone to pick up the work in
order to get their code/series merged.
Which is effectively what you said below, I just wanted to explain why I'm pushing
to remove KVM_GUEST_USES_PFN, and to say that if you or someone else were to write
the code it wouldn't be an automatic nak.
> And yes, the gpc has had bugs as we implemented it, but the point was
> that we got to something which *is* working, and forms a usable
> building block.
>
> So I'm not really sold on the idea of ditching KVM_GUEST_USES_PFN. I
> think we could get it working, and I think it's worth it. But my
> opinion is worth very little unless I express it in 'diff -up' form
> instead of prose, and reverting this particular patch is the least of
> my barriers to doing so, so reluctantly...
next prev parent reply other threads:[~2023-11-27 23:36 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-21 18:02 [PATCH v8 00/15] KVM: xen: update shared_info and vcpu_info handling Paul Durrant
2023-11-21 18:02 ` [PATCH v8 01/15] KVM: pfncache: Add a map helper function Paul Durrant
2023-11-21 18:02 ` [PATCH v8 02/15] KVM: pfncache: remove unnecessary exports Paul Durrant
2023-11-21 21:49 ` David Woodhouse
2023-11-22 8:44 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 03/15] KVM: xen: mark guest pages dirty with the pfncache lock held Paul Durrant
2023-11-21 21:49 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 04/15] KVM: pfncache: add a mark-dirty helper Paul Durrant
2023-11-21 18:02 ` [PATCH v8 05/15] KVM: pfncache: remove KVM_GUEST_USES_PFN usage Paul Durrant
2023-11-21 22:24 ` David Woodhouse
2023-11-27 23:36 ` Sean Christopherson [this message]
2023-11-21 18:02 ` [PATCH v8 06/15] KVM: pfncache: stop open-coding offset_in_page() Paul Durrant
2023-11-21 22:26 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently Paul Durrant
2023-11-21 22:35 ` David Woodhouse
2023-11-22 9:29 ` Paul Durrant
2023-11-22 8:54 ` Xu Yilun
2023-11-22 9:12 ` David Woodhouse
2023-11-22 14:27 ` Xu Yilun
2023-11-22 15:42 ` David Woodhouse
2023-11-22 15:52 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 08/15] KVM: pfncache: allow a cache to be activated with a fixed (userspace) HVA Paul Durrant
2023-11-21 22:47 ` David Woodhouse
2023-11-22 10:07 ` Paul Durrant
2023-11-21 18:02 ` [PATCH v8 09/15] KVM: xen: allow shared_info to be mapped by fixed HVA Paul Durrant
2023-11-21 18:02 ` [PATCH v8 10/15] KVM: xen: allow vcpu_info " Paul Durrant
2023-11-21 18:02 ` [PATCH v8 11/15] KVM: selftests / xen: map shared_info using HVA rather than GFN Paul Durrant
2023-11-21 18:02 ` [PATCH v8 12/15] KVM: selftests / xen: re-map vcpu_info using HVA rather than GPA Paul Durrant
2023-11-21 18:02 ` [PATCH v8 13/15] KVM: xen: advertize the KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA capability Paul Durrant
2023-11-21 18:02 ` [PATCH v8 14/15] KVM: xen: split up kvm_xen_set_evtchn_fast() Paul Durrant
2023-11-21 22:49 ` David Woodhouse
2023-11-21 18:02 ` [PATCH v8 15/15] KVM: xen: allow vcpu_info content to be 'safely' copied Paul Durrant
2023-11-21 22:53 ` David Woodhouse
2023-11-22 10:39 ` David Woodhouse
2023-11-22 10:55 ` Paul Durrant
2023-11-22 11:25 ` David Woodhouse
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