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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Steven Price <steven.price@arm.com>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	kvm@vger.kernel.org, "Radim Krčmář" <rkrcmar@redhat.com>,
	"Marc Zyngier" <maz@kernel.org>,
	"Suzuki K Pouloze" <suzuki.poulose@arm.com>,
	linux-doc@vger.kernel.org, "Russell King" <linux@armlinux.org.uk>,
	linux-kernel@vger.kernel.org, "James Morse" <james.morse@arm.com>,
	"Julien Thierry" <julien.thierry.kdev@gmail.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Will Deacon" <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 04/10] KVM: Implement kvm_put_guest()
Date: Thu, 22 Aug 2019 09:24:49 -0700	[thread overview]
Message-ID: <20190822162449.GF25467@linux.intel.com> (raw)
In-Reply-To: <e2abc69b-74c2-64ef-e270-43d93513eaae@arm.com>

On Thu, Aug 22, 2019 at 04:46:10PM +0100, Steven Price wrote:
> On 22/08/2019 16:28, Sean Christopherson wrote:
> > On Wed, Aug 21, 2019 at 04:36:50PM +0100, Steven Price wrote:
> >> kvm_put_guest() is analogous to put_user() - it writes a single value to
> >> the guest physical address. The implementation is built upon put_user()
> >> and so it has the same single copy atomic properties.
> > 
> > What you mean by "single copy atomic"?  I.e. what guarantees does
> > put_user() provide that __copy_to_user() does not?
> 
> Single-copy atomicity is defined by the Arm architecture[1] and I'm not
> going to try to go into the full details here, so this is a summary.
> 
> For the sake of this feature what we care about is that the value
> written/read cannot be "torn". In other words if there is a read (in
> this case from another VCPU) that is racing with the write then the read
> will either get the old value or the new value. It cannot return a
> mixture. (This is of course assuming that the read is using a
> single-copy atomic safe method).

Thanks for the explanation.  I assumed that's what you were referring to,
but wanted to double check.
 
> __copy_to_user() is implemented as a memcpy() and as such cannot provide
> single-copy atomicity in the general case (the buffer could easily be
> bigger than the architecture can guarantee).
> 
> put_user() on the other hand is implemented (on arm64) as an explicit
> store instruction and therefore is guaranteed by the architecture to be
> single-copy atomic (i.e. another CPU cannot see a half-written value).

I don't think kvm_put_guest() belongs in generic code, at least not with
the current changelog explanation about it providing single-copy atomic
semantics.  AFAICT, the single-copy thing is very much an arm64
implementation detail, e.g. the vast majority of 32-bit architectures,
including x86, do not provide any guarantees, and x86-64 generates more
or less the same code for put_user() and __copy_to_user() for 8-byte and
smaller accesses.

As an alternative to kvm_put_guest() entirely, is it an option to change
arm64's raw_copy_to_user() to redirect to __put_user() for sizes that are
constant at compile time and can be handled by __put_user()?  That would
allow using kvm_write_guest() to update stolen time, albeit with
arguably an even bigger dependency on the uaccess implementation details.

  reply	other threads:[~2019-08-22 16:24 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21 15:36 [PATCH v3 00/10] arm64: Stolen time support Steven Price
2019-08-21 15:36 ` [PATCH v3 01/10] KVM: arm64: Document PV-time interface Steven Price
2019-08-27  8:44   ` Christoffer Dall
2019-08-28 11:23     ` Steven Price
2019-08-27  8:57   ` Christoffer Dall
2019-08-28 12:09     ` Steven Price
2019-08-30  9:22       ` Christoffer Dall
2019-08-28 13:49     ` Christoffer Dall
2019-08-29 15:21       ` Steven Price
2019-08-29 17:15   ` Andrew Jones
2019-08-30  8:35     ` Steven Price
2019-08-21 15:36 ` [PATCH v3 02/10] KVM: arm/arm64: Factor out hypercall handling from PSCI code Steven Price
2019-08-21 15:36 ` [PATCH v3 03/10] KVM: arm64: Implement PV_FEATURES call Steven Price
2019-08-21 15:36 ` [PATCH v3 04/10] KVM: Implement kvm_put_guest() Steven Price
2019-08-22 10:29   ` Jonathan Cameron
2019-08-22 10:37     ` Steven Price
2019-08-22 15:28   ` Sean Christopherson
2019-08-22 15:46     ` Steven Price
2019-08-22 16:24       ` Sean Christopherson [this message]
2019-08-23 10:33         ` Steven Price
2019-08-21 15:36 ` [PATCH v3 05/10] KVM: arm64: Support stolen time reporting via shared structure Steven Price
2019-08-22 10:39   ` Jonathan Cameron
2019-08-22 11:00     ` Steven Price
2019-08-23 12:07   ` Zenghui Yu
2019-08-23 13:23     ` Steven Price
2019-08-21 15:36 ` [PATCH v3 06/10] KVM: Allow kvm_device_ops to be const Steven Price
2019-08-21 15:36 ` [PATCH v3 07/10] KVM: arm64: Provide a PV_TIME device to user space Steven Price
2019-08-22 10:57   ` Jonathan Cameron
2019-08-22 11:11     ` Steven Price
2019-08-22 11:48       ` Jonathan Cameron
2019-08-21 15:36 ` [PATCH v3 08/10] arm/arm64: Provide a wrapper for SMCCC 1.1 calls Steven Price
2019-08-21 15:36 ` [PATCH v3 09/10] arm/arm64: Make use of the SMCCC 1.1 wrapper Steven Price
2019-08-21 15:36 ` [PATCH v3 10/10] arm64: Retrieve stolen time as paravirtualized guest Steven Price
2019-08-23 11:45   ` Zenghui Yu
2019-08-23 14:22     ` Steven Price
2019-08-27 12:43       ` Zenghui Yu

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