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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 14/17] xen/arm64: head: Remove ID map as soon as it is not used
On Mon, 10 Jun 2019, Julien Grall wrote: > The ID map may clash with other parts of the Xen virtual memory layout. > At the moment, Xen is handling the clash by only creating a mapping to > the runtime virtual address before enabling the MMU. > > The rest of the mappings (such as the fixmap) will be mapped after the > MMU is enabled. However, the code doing the mapping is not safe as it > replace mapping without using the Break-Before-Make sequence. > > As the ID map can be anywhere in the memory, it is easier to remove all > the entries added as soon as the ID map is not used rather than adding > the Break-Before-Make sequence everywhere. > > It is difficult to track where exactly the ID map was created without a > full rework of create_page_tables(). Instead, introduce a new function > remove_id_map() will look where is the top-level entry for the ID map > and remove it. > > The new function is only called for the boot CPU. Secondary CPUs will > switch directly to the runtime page-tables so there are no need to > remove the ID mapping. Note that this still doesn't make the Secondary > CPUs path safe but it is not making it worst. > > --- > Note that the comment refers to the patch "xen/arm: tlbflush: Rework > TLB helpers" under review (see [1]). > > Furthermore, it is very likely we will need to re-introduce the ID > map to cater secondary CPUs boot and suspend/resume. For now, the > attempt is to make boot CPU path fully Arm Arm compliant. > > [1] https://lists.xenproject.org/archives/html/xen-devel/2019-05/msg01134.html > --- > xen/arch/arm/arm64/head.S | 86 > ++++++++++++++++++++++++++++++++++++++--------- > 1 file changed, 71 insertions(+), 15 deletions(-) > > diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S > index 192af3e8a2..96e85f8834 100644 > --- a/xen/arch/arm/arm64/head.S > +++ b/xen/arch/arm/arm64/head.S > @@ -300,6 +300,13 @@ real_start_efi: > ldr x0, =primary_switched > br x0 > primary_switched: > + /* > + * The ID map may clash with other parts of the Xen virtual memory > + * layout. As it is not used anymore, remove it completely to > + * avoid having to worry about replacing existing mapping > + * afterwards. > + */ > + bl remove_id_map > bl setup_fixmap > #ifdef CONFIG_EARLY_PRINTK > /* Use a virtual address to access the UART. */ > @@ -632,10 +639,68 @@ enable_mmu: > ret > ENDPROC(enable_mmu) > > +/* > + * Remove the ID map for the page-tables. It is not easy to keep track > + * where the ID map was mapped, so we will look for the top-level entry > + * exclusive to the ID Map and remove it. > + * > + * Inputs: > + * x19: paddr(start) > + * > + * Clobbers x0 - x1 > + */ > +remove_id_map: > + /* > + * Find the zeroeth slot used. Remove the entry from zeroeth > + * table if the slot is not 0. For slot 0, the ID map was either > + * done in first or second table. > + */ > + lsr x1, x19, #ZEROETH_SHIFT /* x1 := zeroeth slot */ > + cbz x1, 1f > + /* It is not in slot 0, remove the entry */ > + ldr x0, =boot_pgtable /* x0 := root table */ > + str xzr, [x0, x1, lsl #3] > + b id_map_removed > + > +1: > + /* > + * Find the first slot used. Remove the entry for the first > + * table if the slot is not 0. For slot 0, the ID map was done > + * in the second table. > + */ > + lsr x1, x19, #FIRST_SHIFT > + and x1, x1, #LPAE_ENTRY_MASK /* x1 := first slot */ > + cbz x1, 1f > + /* It is not in slot 0, remove the entry */ > + ldr x0, =boot_first /* x0 := first table */ > + str xzr, [x0, x1, lsl #3] > + b id_map_removed > + > +1: > + /* > + * Find the second slot used. Remove the entry for the first > + * table if the slot is not 1 (runtime Xen mapping is 2M - 4M). > + * For slot 1, it means the ID map was not created. > + */ > + lsr x1, x19, #SECOND_SHIFT > + and x1, x1, #LPAE_ENTRY_MASK /* x1 := first slot */ > + cmp x1, #1 > + beq id_map_removed > + /* It is not in slot 1, remove the entry */ > + ldr x0, =boot_second /* x0 := second table */ > + str xzr, [x0, x1, lsl #3] Wouldn't it be a bit more reliable if we checked whether the slot in question for x19 (whether zero, first, second) is a pagetable pointer or section map, then zero it if it is a section map, otherwise go down one level? If we did it this way it would be independent from the way create_page_tables is written. With the current code, we are somewhat reliant on the behavior of create_page_tables, because we rely on the position of the slot for the ID map? Where the assumption for instance is that at level one, if the slot is zero, then we need to go down a level, etc. Instead, if we checked if the slot is a section map, we could remove it immediately, if it is a pagetable pointer, we proceed. The code should be similar in complexity and LOC, but it would be more robust. Something like the following, in pseudo-uncompiled assembly: lsr x1, x19, #FIRST_SHIFT ldr x0, =boot_first /* x0 := first table */ ldr x2, [x0, x1, lsl #3] # check x2 against #PT_MEM cbz x2, 1f str xzr, [x0, x1, lsl #3] b id_map_removed > +id_map_removed: > + /* See asm-arm/arm64/flushtlb.h for the explanation of the sequence. > */ Do you mean xen/include/asm-arm/arm64/flushtlb.h? I can't find the explanation you are referring to. > + dsb nshst > + tlbi alle2 > + dsb nsh > + isb > + > + ret > +ENDPROC(remove_id_map) > + > setup_fixmap: > - /* Now we can install the fixmap and dtb mappings, since we > - * don't need the 1:1 map any more */ > - dsb sy > #if defined(CONFIG_EARLY_PRINTK) /* Fixmap is only used by early printk */ > /* Add UART to the fixmap table */ > ldr x1, =xen_fixmap /* x1 := vaddr (xen_fixmap) */ > @@ -653,19 +718,10 @@ setup_fixmap: > ldr x1, =FIXMAP_ADDR(0) > lsr x1, x1, #(SECOND_SHIFT - 3) /* x1 := Slot for FIXMAP(0) */ > str x2, [x4, x1] /* Map it in the fixmap's slot */ > -#endif > > - /* > - * Flush the TLB in case the 1:1 mapping happens to clash with > - * the virtual addresses used by the fixmap or DTB. > - */ > - dsb sy /* Ensure any page table updates made > above > - * have occurred. */ > - > - isb > - tlbi alle2 > - dsb sy /* Ensure completion of TLB flush */ > - isb > + /* Ensure any page table updates made above have occurred */ > + dsb nshst > +#endif > ret > ENDPROC(setup_fixmap) > > -- > 2.11.0 > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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