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charset=UTF-8 Content-Transfer-Encoding: 7bit So, I'm working on kernel shadow stacks. I have some nice optimization code like fred_update_rsp0() for SSP0. It avoids WRMSR's on exits to userspace if FRED_SSP0 hasn't changed, just like FRED_RSP0. Well, turns out that FRED_SSP0 isn't like FRED_RSP0 in one very important way: it is an alias for PL0_SSP, which is managed via XFEATURE_CET_KERNEL which is in: /* Supervisor features which are enabled only in guest FPUs */ #define XFEATURE_MASK_GUEST_SUPERVISOR XFEATURE_MASK_CET_KERNEL So, I was running along just fine in my nice shadow stack kernel going in and out of the kernel just fine. But the first time I did an XRSTOR, the XFEATURE_CET_KERNEL feature got set to its init state. My SSP0 cache didn't notice and I tried to run userspace with SSP0=0. Much hilarity ensued on the next kernel entry. The most adorable part of all this is that I have KVM compiled completely out on my config. The "only in guest FPUs" code is biting me on a kernel that can't even run guests. I think this all started because of 8b05b3c98816 ("x86/fpu/xstate: Add CET supervisor xfeature support as a guest-only feature"), which seems like mostly my doing. So I have mostly myself to blame. Any thoughts on what we should do? Actually context switching XFEATURE_CET_KERNEL seems kinda silly in a lot of ways. PL0_SSP can always be derived _and_ it isn't actually even needed at context switch time, only when running userspace. I could just aggressively blow away my SSP0 cache any time an XRSTOR touches XFEATURE_CET_KERNEL. That would be easy. But it also seems silly that XFEATURE_CET_KERNEL is being blown away at _all_ during normal context switches. Heck, it goes further than that because: fpregs_restore_userregs() does: restore_fpregs_from_fpstate(fpu->fpstate, XFEATURE_MASK_FPSTATE); Where XFEATURE_MASK_FPSTATE contains XFEATURE_MASK_SUPERVISOR_SUPPORTED. So a "restore_userregs" function is blowing away supervisor state. IOW, I think we're using XFEATURE_MASK_FPSTATE a bit too broadly. I'd like to get to a place where only the KVM code even has XFEATURE_CET_KERNEL in its RFBM. Anybody have any bright ideas?