x86/ldt: Simplify the LDT switching logic (73534258) · Commits · 戴 / test

Originally, Linux reloaded the LDT whenever the prev mm or the next mm had an LDT. It was changed in 2002 in: 0bbed3beb4f2 ("[PATCH] Thread-Local Storage (TLS) support") (commit from the historical tree), like this: - /* load_LDT, if either the previous or next thread - * has a non-default LDT. + /* + * load the LDT, if the LDT is different: */ - if (next->context.size+prev->context.size) + if (unlikely(prev->context.ldt != next->context.ldt)) load_LDT(&next->context); The current code is unlikely to avoid any LDT reloads, since different mms won't share an LDT. When we redo lazy mode to stop flush IPIs without switching to init_mm, though, the current logic would become incorrect: it will be possible to have real_prev == next but nonetheless have a stale LDT descriptor. Simplify the code to update LDTR if either the previous or the next mm has an LDT, i.e. effectively restore the historical logic.. While we're at it, clean up the code by moving all the ifdeffery to a header where it belongs. Signed-off-by:

Andy Lutomirski <luto@kernel.org> Reviewed-by:

Thomas Gleixner <tglx@linutronix.de> Reviewed-by:

Borislav Petkov <bp@suse.de> Acked-by:

Rik van Riel <riel@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mel Gorman <mgorman@suse.de> Cc: Nadav Amit <nadav.amit@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: linux-mm@kvack.org Link: http://lkml.kernel.org/r/2a859ac01245f9594c58f9d0a8b2ed8a7cd2507e.1498022414.git.luto@kernel.org Signed-off-by:

Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/mmu_context.h

+26 −0

Original line number	Diff line number	Diff line
		@@ -93,6 +93,32 @@ static inline void load_mm_ldt(struct mm_struct *mm)
		#else
		clear_LDT();
		#endif
		}

		static inline void switch_ldt(struct mm_struct prev, struct mm_struct next)
		{
		#ifdef CONFIG_MODIFY_LDT_SYSCALL
		/*
		* Load the LDT if either the old or new mm had an LDT.
		*
		* An mm will never go from having an LDT to not having an LDT. Two
		* mms never share an LDT, so we don't gain anything by checking to
		* see whether the LDT changed. There's also no guarantee that
		* prev->context.ldt actually matches LDTR, but, if LDTR is non-NULL,
		* then prev->context.ldt will also be non-NULL.
		*
		* If we really cared, we could optimize the case where prev == next
		* and we're exiting lazy mode. Most of the time, if this happens,
		* we don't actually need to reload LDTR, but modify_ldt() is mostly
		* used by legacy code and emulators where we don't need this level of
		* performance.
		*
		* This uses \| instead of \|\| because it generates better code.
		*/
		if (unlikely((unsigned long)prev->context.ldt \|
		(unsigned long)next->context.ldt))
		load_mm_ldt(next);
		#endif

		DEBUG_LOCKS_WARN_ON(preemptible());
		}

arch/x86/mm/tlb.c

+2 −18

Original line number	Diff line number	Diff line
		@@ -148,25 +148,9 @@ void switch_mm_irqs_off(struct mm_struct prev, struct mm_struct next,
		real_prev != &init_mm);
		cpumask_clear_cpu(cpu, mm_cpumask(real_prev));

		/* Load per-mm CR4 state */
		/* Load per-mm CR4 and LDTR state */
		load_mm_cr4(next);

		#ifdef CONFIG_MODIFY_LDT_SYSCALL
		/*
		* Load the LDT, if the LDT is different.
		*
		* It's possible that prev->context.ldt doesn't match
		* the LDT register. This can happen if leave_mm(prev)
		* was called and then modify_ldt changed
		* prev->context.ldt but suppressed an IPI to this CPU.
		* In this case, prev->context.ldt != NULL, because we
		* never set context.ldt to NULL while the mm still
		* exists. That means that next->context.ldt !=
		* prev->context.ldt, because mms never share an LDT.
		*/
		if (unlikely(real_prev->context.ldt != next->context.ldt))
		load_mm_ldt(next);
		#endif
		switch_ldt(real_prev, next);
		}

		/*

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