On Fri, Nov 24, 2017 at 10:14:27AM +0100, Ingo Molnar wrote: > Interactions with SWAPGS: previous versions of the KAISER code > relied on having per-cpu scratch space to save/restore a register > that can be used for the CR3 MOV. The %GS register is used to > index into our per-cpu space, so SWAPGS *had* to be done before > the CR3 switch. That scratch space is gone now, but the semantic > that SWAPGS must be done before the CR3 MOV is retained. This is > good to keep because it is not that hard to do and it allows us > to do things like add per-cpu debugging information to help us > figure out what goes wrong sometimes.
> +.macro SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg:req save_reg:req > + movq %cr3, %r\scratch_reg > + movq %r\scratch_reg, \save_reg > + /* > + * Is the switch bit zero? This means the address is > + * up in real KAISER patches in a moment. > + */ > + testq $(KAISER_SWITCH_MASK), %r\scratch_reg > + jz .Ldone_\@ > + > + ADJUST_KERNEL_CR3 %r\scratch_reg > + movq %r\scratch_reg, %cr3 > + > +.Ldone_\@: > +.endm > @@ -1333,6 +1362,7 @@ ENTRY(error_entry) > * gsbase and proceed. We'll fix up the exception and land in > * .Lgs_change's error handler with kernel gsbase. > */ > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rax > SWAPGS > jmp .Lerror_entry_done > > @@ -1343,9 +1373,10 @@ ENTRY(error_entry) > > .Lerror_bad_iret: > /* > - * We came from an IRET to user mode, so we have user gsbase. > - * Switch to kernel gsbase: > + * We came from an IRET to user mode, so we have user > + * gsbase and CR3. Switch to kernel gsbase and CR3: > */ > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rax > SWAPGS > > /* The Changelog states SWAPGS must be done before, yet the code does after.
