Subject: Re: [PATCH v3 04/10] KVM: Implement kvm_put_guest()

From: Sean Christopherson <[email-masked]>
Date: Thu, 22 Aug 2019 09:24:49 -0700

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.

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