On Fri, Nov 24, 2017 at 5:47 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Nov 24, 2017 at 10:14:35AM +0100, Ingo Molnar wrote: >> From: Dave Hansen <dave.han...@linux.intel.com> >> >> There is some rather arcane code to help when an IRET returns >> to 16-bit segments. It is referred to as the "espfix" code. >> This consists of a few per-cpu variables: >> >> espfix_stack: tells us where the stack is allocated >> (the bottom) >> espfix_waddr: tells us to where %rsp may be pointed >> (the top) >> >> These are in addition to the stack itself. All three things must >> be mapped for the espfix code to function. >> >> Note: the espfix code runs with a kernel GSBASE, but user >> (shadow) page tables. A switch to the kernel page tables could >> be performed instead of mapping these structures, but mapping >> them is simpler and less likely to break the assembly. To switch >> over to the kernel copy, additional temporary storage would be >> required which is in short supply in this context. > > With Andy's patches that should actually be doable, no?
I don't think it has much to do with my patches. We can freely spill to the stack in the espfix64 code, though. --Andy
