On 22/08/2019 17:24, Sean Christopherson wrote: > 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. I think it's important to in some way ensure that the desire that this is a single write is shown. copy_to_user() is effectively "setup();memcpy();finish();" and while a good memcpy() implementation would be identical to put_user() there's a lot more room for this being broken in the future by changes to the memcpy() implementation. (And I don't want to require that memcpy() has to detect this case). One suggestion is to call it something like kvm_put_guest_atomic() to reflect the atomicity requirement. Presumably that would be based on a new put_user_atomic() which architectures could override as necessary if put_user() doesn't provide the necessary guarantees. Steve
