>>> On 17.10.13 at 11:41, Gleb Natapov <gleb@xxxxxxxxxx> wrote: > On Thu, Oct 17, 2013 at 10:33:33AM +0100, Jan Beulich wrote: >> >>> On 17.10.13 at 11:27, Gleb Natapov <gleb@xxxxxxxxxx> wrote: >> > On Wed, Oct 16, 2013 at 05:13:40PM +0100, Jan Beulich wrote: >> >> > It preserves *less* state, because the upper 32 bits of rip are now >> >> > corrupted. Any 64-bit application that actually looks at the FP >> >> > rip/rdp fields now get the WRONG VALUES. >> >> >> >> But again - this isn't being done for ordinary 64-bit applications, >> >> this is only happening for KVM guests. And there not being a >> >> protocol for telling the caller whether a certain context hold >> >> 64-bit offsets or selector/offset pairs shouldn't be a reason to >> >> think of a solution to the problem. >> >> >> > KVM knows what mode guest vcpu is in. is_long_mode(vcpu) will tell you >> > if it is in long mode or not. No need to guess it. >> >> So what if that 64-bit guest OS is running a 32-bit app? You can >> only positively know the _current_ guest word size when the >> guest is not in long mode. >> > 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. 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
