On 03/24/2015 04:52 PM, Andy Lutomirski wrote: > On Tue, Mar 24, 2015 at 4:42 PM, Dave Hansen <[email protected]> wrote: >> On 03/24/2015 03:28 PM, Andy Lutomirski wrote: >>> Your function appears to be getting it for write (I assume that's what >>> the unlazy_fpu is for), so I'd rather have it called >>> tsk_get_xsave_field_for_write or something like that. >> >> It should be entirely read-only. >> >> For MPX (the only user of get_xsave_addr() iirc), we are only worried >> about getting the status codes (and addresses) out of the bndstatus >> register and making sure that the kernel-recorded bounds directory >> address matches the bndcfgu (configuration) register. >> >> We don't ever write to the registers. > > So why are you unlazying it?
Oleg actually suggested it. > IIUC, the xstae for current can be in one of three logical states: > > 1. Live in CPU regs. The in-memory copy is garbage and the state is > in CPU regs. > 2. Lazy. The in-memory copy and the CPU regs match. Writing to > either copy is illegal. > 3. In memory only. Writing to the in-memory copy is safe. > > IIUC, you want to read the xstate, do you're okay with #2 or #3. This > would be tsk_get_xsave_field_for_read in my terminology. > > If you want to write the xstate, you'd need to be in state #3, which > would be tsk_get_xsave_field_for_write. > > IIUC, unlazy_fpu just moves from from state 2 to 3. I won't completely claim to understand what's going on with the FPU code, but I think your analysis is a bit off. unlazy_fpu() does __save_init_fpu() which (among other things) calls xsave to dump the CPU registers to memory. That doesn't make any sense to do if "The in-memory copy and the CPU regs match." IOW, unlazy_fpu() is called when the in-memory copy is garbage and takes us to a state where we can look at the in-memory copy. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
