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a="1560749" X-IronPort-AV: E=Sophos;i="6.27,175,1787036400"; d="scan'208";a="1560749" Received: from orviesa001.jf.intel.com ([10.64.159.141]) by orvoesa110.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Oct 2026 14:21:28 -0700 X-CSE-ConnectionGUID: Gv6hCAr5QASp3dkFzOplRw== X-CSE-MsgGUID: 0vcg4BatQJqaHOyfMP8IXw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.27,175,1787036400"; d="scan'208";a="3669481" Received: from vverma7-desk1.amr.corp.intel.com (HELO [10.125.108.4]) ([10.125.108.4]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Oct 2026 14:21:22 -0700 Message-ID: <5c74f231-46a7-4f37-8fcd-bff84842a923@intel.com> Date: Fri, 23 Oct 2026 14:21:20 -0700 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Kernel Shadow Stacks Versus XFEATURE_MASK_GUEST_SUPERVISOR To: Paolo Bonzini , Yang Weijiang , "Gao, Chao" , "Edgecombe, Rick P" , Maxim Levitsky , John Allen , LKML , peterz@infradead.org, kvm@vger.kernel.org, binbin.wu@linux.intel.com, Andrew Cooper , Thomas Gleixner , "Bae, Chang Seok" , "Mehta, Sohil" References: <2f0560df-69fc-4fc1-81fa-447e289ec2df@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/23/26 14:00, Paolo Bonzini wrote: > On 10/23/26 21:03, Dave Hansen wrote: ... > If you want to quickly work around for your !CONFIG_KVM kernel, I would do > > #ifdef CONFIG_KVM > /* Supervisor features which are enabled only in guest FPUs */ > #define XFEATURE_MASK_GUEST_SUPERVISOR  XFEATURE_MASK_CET_KERNEL > #else > #define XFEATURE_MASK_GUEST_SUPERVISOR  0 > #endif Yup, that'd work for sure. Thanks! >> 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. > > True but, just to understand, what is the performance like if you > actually use XSAVES/XRSTORS to update SSP0 on context switches instead > of WRMSR+cache?  Hopefully the microcode does a bunch of optimizations > as well. First, we can use WRMSRNS for this MSR, so it's faster than usual. Second, on kernel=>kernel context switches that don't go out to userspace, it doesn't need to be written at all. It's kinda like the user FPU state. Third, I'm actually reusing the fred_ssp0 cache, so further reduces the incremental cost. Fourth, it's not _really_ context-switched. There's never a need to read the register. It can always be conjured out of thin air. It's possible to only write the register, never read it. I can benchmark it, but I seriously doubt that an XSAVE/XRSTOR at context switch of three fields is going to beat an WRMSRNS of one. >> 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. > > The actual mask used by restore_fpregs_from_fpstate is tighter though, > it's "fpu_kernel_cfg.max_features & XFEATURE_MASK_FPSTATE". > > Can you replace fpu_kernel_cfg.max_features with some fpstate->xyz > (sorry too late to think it through :)) that has CET_KERNEL for the > guest FPU and not for the regular one? Sure I can give that a shot. There's one minor issue in that we need to init dynamic features which _aren't_ in fpstate->xfeatures when context switching. There's a little note about this in restore_fpregs_from_fpstate(). I think what you're getting at is that we need _something_ to say that KVM fpstates should muck with XFEATURE_CET_KERNEL and task fpstates should not, even though the task ones have space for XFEATURE_CET_KERNEL. That's certainly doable. >> 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. > That I agree, but I think it can be handled in {save,restore} > _fpregs_to_fpstate(). I think I understand what you're going after at least. I'll poke at it a bit more. Thanks for the thoughts!