Re: [PATCH 2/4] xen/arm: acpi: The fixmap area should always be cleared during failure/unmap

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 26 Sep 2020, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> Commit 022387ee1ad3 "xen/arm: mm: Don't open-code Xen PT update in
> {set, clear}_fixmap()" enforced that each set_fixmap() should be
> paired with a clear_fixmap(). Any failure to follow the model would
> result to a platform crash.
> 
> Unfortunately, the use of fixmap in the ACPI code was overlooked as it
> is calling set_fixmap() but not clear_fixmap().
> 
> The function __acpi_os_map_table() is reworked so:
>     - We know before the mapping whether the fixmap region is big
>     enough for the mapping.
>     - It will fail if the fixmap is always inuse.

I take you mean "it will fail if the fixmap is *already* in use"?

If so, can it be a problem? Or the expectation is that in practice
__acpi_os_map_table() will only get called once before SYS_STATE_boot?

Looking at the code it would seem that even before this patch
__acpi_os_map_table() wasn't able to handle multiple calls before
SYS_STATE_boot.


> 
> The function __acpi_os_unmap_table() will now call clear_fixmap().
> 
> Reported-by: Wei Xu <xuwei5@xxxxxxxxxxxxx>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
> 
> The discussion on the original thread [1] suggested to also zap it on
> x86. This is technically not necessary today, so it is left alone for
> now.
> 
> I looked at making the fixmap code common but the index are inverted
> between Arm and x86.
> 
> [1] https://lore.kernel.org/xen-devel/5E26C935.9080107@xxxxxxxxxxxxx/
> ---
>  xen/arch/arm/acpi/lib.c | 75 +++++++++++++++++++++++++++++++----------
>  1 file changed, 58 insertions(+), 17 deletions(-)
> 
> diff --git a/xen/arch/arm/acpi/lib.c b/xen/arch/arm/acpi/lib.c
> index 2192a5519171..eebaca695562 100644
> --- a/xen/arch/arm/acpi/lib.c
> +++ b/xen/arch/arm/acpi/lib.c
> @@ -25,38 +25,79 @@
>  #include <xen/init.h>
>  #include <xen/mm.h>
>  
> +static bool fixmap_inuse;
> +
>  char *__acpi_map_table(paddr_t phys, unsigned long size)
>  {
> -    unsigned long base, offset, mapped_size;
> -    int idx;
> +    unsigned long base, offset;
> +    mfn_t mfn;
> +    unsigned int idx;
>  
>      /* No arch specific implementation after early boot */
>      if ( system_state >= SYS_STATE_boot )
>          return NULL;
>  
>      offset = phys & (PAGE_SIZE - 1);
> -    mapped_size = PAGE_SIZE - offset;
> -    set_fixmap(FIXMAP_ACPI_BEGIN, maddr_to_mfn(phys), PAGE_HYPERVISOR);
> -    base = FIXMAP_ADDR(FIXMAP_ACPI_BEGIN);
> +    base = FIXMAP_ADDR(FIXMAP_ACPI_BEGIN) + offset;
> +
> +    /* Check the fixmap is big enough to map the region */
> +    if ( (FIXMAP_ADDR(FIXMAP_ACPI_END) + PAGE_SIZE - base) < size )
> +        return NULL;
> +
> +    /* With the fixmap, we can only map one region at the time */
> +    if ( fixmap_inuse )
> +        return NULL;
>  
> -    /* Most cases can be covered by the below. */
> +    fixmap_inuse = true;
> +
> +    size += offset;
> +    mfn = maddr_to_mfn(phys);
>      idx = FIXMAP_ACPI_BEGIN;
> -    while ( mapped_size < size )
> -    {
> -        if ( ++idx > FIXMAP_ACPI_END )
> -            return NULL;    /* cannot handle this */
> -        phys += PAGE_SIZE;
> -        set_fixmap(idx, maddr_to_mfn(phys), PAGE_HYPERVISOR);
> -        mapped_size += PAGE_SIZE;
> -    }
>  
> -    return ((char *) base + offset);
> +    do {
> +        set_fixmap(idx, mfn, PAGE_HYPERVISOR);
> +        size -= min(size, (unsigned long)PAGE_SIZE);
> +        mfn = mfn_add(mfn, 1);
> +        idx++;
> +    } while ( size > 0 );
> +
> +    return (char *)base;
>  }
>  
>  bool __acpi_unmap_table(void *ptr, unsigned long size)
>  {
> -    return ( vaddr >= FIXMAP_ADDR(FIXMAP_ACPI_BEGIN) &&
> -             vaddr < (FIXMAP_ADDR(FIXMAP_ACPI_END) + PAGE_SIZE) );
> +    vaddr_t vaddr = (vaddr_t)ptr;
> +    unsigned int idx;
> +
> +    /* We are only handling fixmap address in the arch code */
> +    if ( vaddr < FIXMAP_ADDR(FIXMAP_ACPI_BEGIN) ||
> +         vaddr >= FIXMAP_ADDR(FIXMAP_ACPI_END) )

The "+ PAGE_SIZE" got lost


> +        return false;
> +
> +    /*
> +     * __acpi_map_table() will always return a pointer in the first page
> +     * for the ACPI fixmap region. The caller is expected to free with
> +     * the same address.
> +     */
> +    ASSERT((vaddr & PAGE_MASK) == FIXMAP_ADDR(FIXMAP_ACPI_BEGIN));
> +
> +    /* The region allocated fit in the ACPI fixmap region. */
> +    ASSERT(size < (FIXMAP_ADDR(FIXMAP_ACPI_END) + PAGE_SIZE - vaddr));
> +    ASSERT(fixmap_inuse);
> +
> +    fixmap_inuse = false;
> +
> +    size += FIXMAP_ADDR(FIXMAP_ACPI_BEGIN) - vaddr;

Sorry I got confused.. Shouldn't this be:

  size += vaddr - FIXMAP_ADDR(FIXMAP_ACPI_BEGIN);

?


> +    idx = FIXMAP_ACPI_BEGIN;
> +
> +    do
> +    {
> +        clear_fixmap(idx);
> +        size -= min(size, (unsigned long)PAGE_SIZE);
> +        idx++;
> +    } while ( size > 0 );
> +
> +    return true;
>  }
>  
>  /* True to indicate PSCI 0.2+ is implemented */
> -- 
> 2.17.1
>