Messages in this thread | | | Date | Thu, 27 May 2010 18:10:14 -1000 | From | Zachary Amsden <> | Subject | Re: [PATCH] Add Documentation/kvm/msr.txt |
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On 05/27/2010 02:10 PM, Glauber Costa wrote: > On Thu, May 27, 2010 at 11:02:35AM -1000, Zachary Amsden wrote: > >> On 05/27/2010 10:36 AM, Glauber Costa wrote: >> >>> On Thu, May 27, 2010 at 10:13:12AM -1000, Zachary Amsden wrote: >>> >>>> On 05/27/2010 06:02 AM, Glauber Costa wrote: >>>> >>>>> On Thu, May 27, 2010 at 11:15:43AM +0300, Avi Kivity wrote: >>>>> >>>>>> On 05/26/2010 09:04 PM, Glauber Costa wrote: >>>>>> >>>>>>> This patch adds a file that documents the usage of KVM-specific >>>>>>> MSRs. >>>>>>> >>>>>>> >>>>>> Looks good. A few comments: >>>>>> >>>>>> >>>>>>> + >>>>>>> +Custom MSR list >>>>>>> +-------- >>>>>>> + >>>>>>> +The current supported Custom MSR list is: >>>>>>> + >>>>>>> +MSR_KVM_WALL_CLOCK: 0x11 >>>>>>> + >>>>>>> + data: physical address of a memory area. >>>>>>> >>>>>> Which must be in guest RAM (i.e., don't point it somewhere random >>>>>> and expect the hypervisor to allocate it for you). >>>>>> >>>>>> Must be aligned to 4 bytes (we don't enforce it though). >>>>>> >>>>> I don't see the reason for it. >>>>> >>>>> If this is a requirement, our own implementation >>>>> is failing to meet it. >>>>> >>>> It's so the atomic write actually is atomic. >>>> >>> Which atomic write? This is the wallclock, we do no atomic writes for >>> querying it. Not to confuse with system time (the other msr). >>> >>> >>>> Stating a 4 -byte >>>> alignment requirement prevents the wall clock from crossing a page >>>> boundary. >>>> >>> Yes, but why require it? >>> >>> reading the wallclock is not a hot path for anybody, is usually done >>> just once, and crossing a page boundary here does not pose any correctness >>> issue. >>> >> Little-endian non-atomic page crossing writes will write the small >> part of the wallclock first, so another CPU may observe the >> following wallclock sequence: >> >> 0x01ff .. 0x0100 .. 0x0200 >> >> Big-endian writes also have similar failure: >> >> 0x01ff .. 0x02ff .. 0x0200 >> >> This won't happen if there is a single instruction write of the wall >> clock word. >> > We already specify that users can only trust the value of the wallclock > after we have an even version field. > When we start the update, and during the time of all writes to it, > it is odd, and thus, invalid. > The ABI guarantees to the guest that we'll only bump version > after we're done updating. >
I guess there is not a point.
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