linux-kernel - Re: [PATCH v13 34/85] KVM: Get writable mapping for __kvm_vcpu

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Message-ID: <ZxaZtJGfN2_5Db6h@google.com>
Date: Mon, 21 Oct 2024 11:13:08 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Tianrui Zhao <zhaotianrui@...ngson.cn>, 
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>, 
	Michael Ellerman <mpe@...erman.id.au>, Anup Patel <anup@...infault.org>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	"Alex Bennée" <alex.bennee@...aro.org>, David Matlack <dmatlack@...gle.com>, 
	David Stevens <stevensd@...omium.org>, Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH v13 34/85] KVM: Get writable mapping for __kvm_vcpu_map()
 only when necessary

On Mon, Oct 21, 2024, Yan Zhao wrote:
> On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote:
> > When creating a memory map for read, don't request a writable pfn from the
> > primary MMU.  While creating read-only mappings can be theoretically slower,
> > as they don't play nice with fast GUP due to the need to break CoW before
> > mapping the underlying PFN, practically speaking, creating a mapping isn't
> > a super hot path, and getting a writable mapping for reading is weird and
> > confusing.
> > 
> > Tested-by: Alex Bennée <alex.bennee@...aro.org>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  virt/kvm/kvm_main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 080740f65061..b845e9252633 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
> >  	struct kvm_follow_pfn kfp = {
> >  		.slot = gfn_to_memslot(vcpu->kvm, gfn),
> >  		.gfn = gfn,
> > -		.flags = FOLL_WRITE,
> > +		.flags = writable ? FOLL_WRITE : 0,
> >  		.refcounted_page = &map->pinned_page,
> >  		.pin = true,
> >  	};
> When writable is false, could we set ".pin = false," ?

Hmm, maybe?  I can't imagine anything would actually break, but unless FOLL_PIN
implies writing, my preference would still be to pin the page so that KVM always
pins when accessing the actual data of a page.

> Also not sure if ".map_writable = NULL" is missing.

Doh, my previous response was slightly wrong, it's implicitly initialized to NULL,
not false.  I forgot map_writable is a pointer to a bool.