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SubjectRe: [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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