Messages in this thread | | | Date | Sat, 24 Jan 2015 08:39:25 -0500 | From | Rik van Riel <> | Subject | Re: question about save_xstate_sig() - WHY DOES THIS WORK? |
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On 01/23/2015 04:07 PM, H. Peter Anvin wrote: > On 01/23/2015 11:34 AM, Rik van Riel wrote: >> While working on a patch series to defer FPU state loading until >> kernel -> user space transition, and be more lazy with FPU state >> while in the kernel, I came across this code in >> save_xstate_sig(). >> >> Not only is this broken with my new code, but it looks like it >> may be broken with the current code, too... >> >> Specifically, save_user_xstate() may page fault and sleep. After >> returning from the page fault, there is no guarantee that the FPU >> state will be restored into the CPU, when the system is not >> running with eager fpu mode. >> >> In that case, what prevents us from saving random FPU register >> state to the user's stack frame? Potentially state containing >> data from other programs... >> > > If the FPU state is not current, we'll have CR0.TS = 1 and the > XSAVE will cause an #NM exception, which will cause the FPU state > to be swapped in.
OK, so the code works as it is right now, but I still need to have the patch in my "defer FPU state loading and CR0.TS switching to kernel -> user space transition" patch series.
Thanks for explaining why the current code is safe, Peter.
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