Re: [Xen-devel] [PATCH v3 02/28] xen/arm64: head: Remove 1:1 mapping as soon as it is not used

[Stefano Stabellini <sstabellini@kernel.org>]

On Mon, 12 Aug 2019, Julien Grall wrote:
> The 1:1 mapping 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 1:1 mapping can be anywhere in the memory, it is easier to remove
> all the entries added as soon as the 1:1 mapping is not used rather than
> adding the Break-Before-Make sequence everywhere.
> 
> It is difficult to track where exactly the 1:1 mapping was created
> without a full rework of create_page_tables(). Instead, introduce a new
> function remove_identity_mapping() will look where is the top-level entry
> for the 1:1 mapping 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 1:1 mapping. Note that this still doesn't make the Secondary
> CPUs path safe but it is not making it worst.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> 
> ---
>     It is very likely we will need to re-introduce the 1:1 mapping to cater
>     secondary CPUs boot and suspend/resume. For now, the attempt is to make
>     boot CPU path fully Arm Arm compliant.
> 
>     Changes in v3:
>         - Avoid hardcoding slots
> 
>     Changes in v2:
>         - s/ID map/1:1 mapping/
>         - Rename remove_id_map() to remove_identity_mapping()
>         - Add missing signed-off-by
> ---
>  xen/arch/arm/arm64/head.S | 94 +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 79 insertions(+), 15 deletions(-)
> 
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 50cff08756..ec138aae3e 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -33,6 +33,11 @@
>  #define PT_DEV    0xe71 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=0 P=1 */
>  #define PT_DEV_L3 0xe73 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=1 P=1 */
>  
> +/* Convenience defines to get slot used by Xen mapping. */
> +#define XEN_ZEROETH_SLOT    zeroeth_table_offset(XEN_VIRT_START)
> +#define XEN_FIRST_SLOT      first_table_offset(XEN_VIRT_START)
> +#define XEN_SECOND_SLOT     second_table_offset(XEN_VIRT_START)
> +
>  #define __HEAD_FLAG_PAGE_SIZE   ((PAGE_SHIFT - 10) / 2)
>  
>  #define __HEAD_FLAG_PHYS_BASE   1
> @@ -301,6 +306,13 @@ real_start_efi:
>          ldr   x0, =primary_switched
>          br    x0
>  primary_switched:
> +        /*
> +         * The 1:1 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_identity_mapping
>          bl    setup_fixmap
>  #ifdef CONFIG_EARLY_PRINTK
>          /* Use a virtual address to access the UART. */
> @@ -626,10 +638,71 @@ enable_mmu:
>          ret
>  ENDPROC(enable_mmu)
>  
> +/*
> + * Remove the 1:1 map for the page-tables. It is not easy to keep track
                         ^ from

> + * where the 1:1 map was mapped, so we will look for the top-level entry
> + * exclusive to the 1:1 map and remove it.
> + *
> + * Inputs:
> + *   x19: paddr(start)
> + *
> + * Clobbers x0 - x1
> + */
> +remove_identity_mapping:
> +        /*
> +         * Find the zeroeth slot used. Remove the entry from zeroeth
> +         * table if the slot is not XEN_ZEROETH_SLOT. 

This part of the comment is good


> +         * For slot XEN_ZEROETH_SLOT, the 1:1 mapping was either done in first
> +         * or second table.

I don't think this sentence is very useful now that the slot is not
hard-coded anymore. I would remove it. Instead, I would write something
like "The slot XEN_ZEROETH_SLOT is used for the XEN_VIRT_START mapping."
The same goes for all the other levels.


> +         */
> +        lsr   x1, x19, #ZEROETH_SHIFT   /* x1 := zeroeth slot */
> +        cmp   x1, #XEN_ZEROETH_SLOT
> +        beq   1f
> +        /* It is not in slot XEN_ZEROETH_SLOT, remove the entry. */
> +        ldr   x0, =boot_pgtable         /* x0 := root table */
> +        str   xzr, [x0, x1, lsl #3]
> +        b     identity_mapping_removed
> +
> +1:
> +        /*
> +         * Find the first slot used. Remove the entry for the first
> +         * table if the slot is not XEN_FIRST_SLOT. For slot XEN_FIRST_SLOT,
> +         * the 1:1 mapping was done in the second table.
> +         */
> +        lsr   x1, x19, #FIRST_SHIFT
> +        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
> +        cmp   x1, #XEN_FIRST_SLOT
> +        beq   1f
> +        /* It is not in slot XEN_FIRST_SLOT, remove the entry. */
> +        ldr   x0, =boot_first           /* x0 := first table */
> +        str   xzr, [x0, x1, lsl #3]
> +        b     identity_mapping_removed
> +
> +1:
> +        /*
> +         * Find the second slot used. Remove the entry for the first
> +         * table if the slot is not XEN_SECOND_SLOT. For slot XEN_SECOND_SLOT,
> +         * it means the 1:1 mapping was not created.
> +         */
> +        lsr   x1, x19, #SECOND_SHIFT
> +        and   x1, x1, #LPAE_ENTRY_MASK  /* x1 := first slot */
> +        cmp   x1, #XEN_SECOND_SLOT
> +        beq   identity_mapping_removed
> +        /* It is not in slot 1, remove the entry */
> +        ldr   x0, =boot_second          /* x0 := second table */
> +        str   xzr, [x0, x1, lsl #3]
> +
> +identity_mapping_removed:
> +        /* See asm-arm/arm64/flushtlb.h for the explanation of the sequence. */
> +        dsb   nshst
> +        tlbi  alle2
> +        dsb   nsh
> +        isb

I just want to point out that asm-arm/arm64/flushtlb.h says to use:

 * DSB ISHST        // Ensure prior page-tables updates have completed
 * TLBI...          // Invalidate the TLB
 * DSB ISH          // Ensure the TLB invalidation has completed
 * ISB              // See explanation below

Also implemented as:

    asm volatile(               \
        "dsb  ishst;"           \
        "tlbi "  # tlbop  ";"   \
        "dsb  ish;"             \
        "isb;"                  \
        : : : "memory");        \

Why is non-shareable enough? Shouldn't it be inner shareable?


> +        ret
> +ENDPROC(remove_identity_mapping)
> +
>  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) */
> @@ -647,19 +720,10 @@ 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

Same here about shareability


> +#endif
>          ret
>  ENDPROC(setup_fixmap)
>  
> -- 
> 2.11.0
> 

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