linux-kernel - Re: [PATCH 10/19] KVM: Use follow

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Message-ID: <ZrpbP9Ow9EcpQtCF@x1n>
Date: Mon, 12 Aug 2024 14:58:07 -0400
From: Peter Xu <peterx@...hat.com>
To: Axel Rasmussen <axelrasmussen@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Jason Gunthorpe <jgg@...dia.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Borislav Petkov <bp@...en8.de>,
	David Hildenbrand <david@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 10/19] KVM: Use follow_pfnmap API

On Fri, Aug 09, 2024 at 10:23:20AM -0700, Axel Rasmussen wrote:
> On Fri, Aug 9, 2024 at 9:09 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > Use the new pfnmap API to allow huge MMIO mappings for VMs.  The rest work
> > is done perfectly on the other side (host_pfn_mapping_level()).
> 
> I don't think it has to be done in this series, but a future
> optimization to consider is having follow_pfnmap just tell the caller
> about the mapping level directly. It already found this information as
> part of its walk. I think there's a possibility to simplify KVM /
> avoid it having to do its own walk again later.

AFAIU pfnmap isn't special in this case, as we do the "walk pgtable twice"
idea also to a generic page here, so probably not directly relevant to this
patch alone.

But I agree with you, sounds like something we can consider trying.  I
would be curious on whether the perf difference would be measurable in this
specific case, though.  I mean, this first walk will heat up all the
things, so I'd expect the 2nd walk (which is lockless) later be pretty fast
normally.

Thanks,

-- 
Peter Xu