Re: [RFC PATCH 12/15] kmap: Add stray write protection for device pages

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 14, 2020 at 12:02:17AM -0700, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
> 
> Device managed pages may have additional protections.  These protections
> need to be removed prior to valid use by kernel users.
> 
> Check for special treatment of device managed pages in kmap and take
> action if needed.  We use kmap as an interface for generic kernel code
> because under normal circumstances it would be a bug for general kernel
> code to not use kmap prior to accessing kernel memory.  Therefore, this
> should allow any valid kernel users to seamlessly use these pages
> without issues.
> 
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> ---
>  include/linux/highmem.h | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index d6e82e3de027..7f809d8d5a94 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -8,6 +8,7 @@
>  #include <linux/mm.h>
>  #include <linux/uaccess.h>
>  #include <linux/hardirq.h>
> +#include <linux/memremap.h>
>  
>  #include <asm/cacheflush.h>
>  
> @@ -31,6 +32,20 @@ static inline void invalidate_kernel_vmap_range(void *vaddr, int size)
>  
>  #include <asm/kmap_types.h>
>  
> +static inline void enable_access(struct page *page)
> +{
> +	if (!page_is_access_protected(page))
> +		return;
> +	dev_access_enable();
> +}
> +
> +static inline void disable_access(struct page *page)
> +{
> +	if (!page_is_access_protected(page))
> +		return;
> +	dev_access_disable();
> +}

So, if I followed along correctly, you're proposing to do a WRMSR per
k{,un}map{_atomic}(), sounds like excellent performance all-round :-(