From: Steven Price <steven.price@arm.com>
To: Sean Christopherson <sean.j.christopherson@intel.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 16:46:10 +0100 [thread overview]
Message-ID: <e2abc69b-74c2-64ef-e270-43d93513eaae@arm.com> (raw)
In-Reply-To: <20190822152854.GE25467@linux.intel.com>
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).
__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).
Steve
[1] https://static.docs.arm.com/ddi0487/ea/DDI0487E_a_armv8_arm.pdf#page=110
next prev parent reply other threads:[~2019-08-22 15:46 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 [this message]
2019-08-22 16:24 ` Sean Christopherson
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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