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Date: Wed, 1 Nov 2017 22:25:26 +0100 (CET) From: Thomas Gleixner <tglx@...utronix.de> To: Dave Hansen <dave.hansen@...ux.intel.com> cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at, michael.schwarz@...k.tugraz.at, luto@...nel.org, torvalds@...ux-foundation.org, keescook@...gle.com, hughd@...gle.com, x86@...nel.org Subject: Re: [PATCH 04/23] x86, tlb: make CR4-based TLB flushes more robust On Tue, 31 Oct 2017, Dave Hansen wrote: > Our CR4-based TLB flush currently requries global pages to be > supported *and* enabled. But, we really only need for them to be > supported. Make the code more robust by alllowing X86_CR4_PGE to > clear as well as set. That's not what the patch is actually doing. > cr4 = this_cpu_read(cpu_tlbstate.cr4); > - /* clear PGE */ > - native_write_cr4(cr4 & ~X86_CR4_PGE); > - /* write old PGE again and flush TLBs */ > + /* > + * This function is only called on systems that support X86_CR4_PGE > + * and where always set X86_CR4_PGE. Warn if we are called without > + * PGE set. > + */ > + WARN_ON_ONCE(!(cr4 & X86_CR4_PGE)); Because if CR4_PGE is not set, this warning triggers. So this defeats the toggle mode you are implementing. > + /* > + * Architecturally, any _change_ to X86_CR4_PGE will fully flush the > + * TLB of all entries including all entries in all PCIDs and all > + * global pages. Make sure that we _change_ the bit, regardless of > + * whether we had X86_CR4_PGE set in the first place. > + */ > + native_write_cr4(cr4 ^ X86_CR4_PGE); > + /* Put original CR3 value back: */ That want's to be CR4. Restoring CR3 to CR4 might be suboptimal. > native_write_cr4(cr4); Thanks, tglx
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