>>> On 17.10.13 at 12:23, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > On Thu, Oct 17, 2013 at 10:51:52AM +0100, Jan Beulich wrote: >> >>> On 17.10.13 at 11:41, Gleb Natapov <gleb@xxxxxxxxxx> wrote: >> > KVM obviously knows the complete state of virtual CPU. It can figure the >> > situation above by looking at CS descriptor, not need to check >> > is_long_mode() at all. Here is how emulator does it: >> >> And again - no: The last floating point operation may have >> happened in 32-bit user mode context, while the state saving >> may happen when the guest is already back in 64-bit kernel >> mode. >> > Hmm, OK so the scenarios you are talking about is: > 1. Guest's 32bit process uses FPU > 2. Guest switch to 64bit kernel. > 3. Before guest's kernel saves 32bit process's FPU state VMEXIT happens > 4. KVM need to save FPU but it does not know what mode it is in > Correct? Yes. > KVM gives FPU to a guest lazily, meaning that on a first FPU use #NM > (intercepted by KVM) happens at which point FPU is granted to a guest. > KVM can check what mode CPU was in at this point and use this info > while saving FPU. But there is additional optimization that will prevent > this from working for all cases: when FPU is granted to a guest KVM > disabled CR0.TS/#NM intercepts, so guest is free to switch FPU from > 32bit to 64bit mode without KVM knowing. Disabling this optimization > will make FP intensive workload slow in a guest. Not sure what you're trying to tell me with this explanation. Jan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
