linux-kernel - Re: [PATCH 6/6] kvm, faster and simpler version of get_user_page_and

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Date:	Sun, 18 Jul 2010 17:18:04 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>, Nick Piggin <npiggin@...e.de>
CC:	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 6/6] kvm, faster and simpler version of get_user_page_and_protection()

On 07/16/2010 05:13 AM, Lai Jiangshan wrote:
> a light weight version of get_user_page_and_protection()
>
>
> @@ -375,3 +374,83 @@ slow_irqon:
>   		return ret;
>   	}
>   }
> +
> +/*
> + * get a current mapped page fast, and test whether the page is writable.
> + * equivalent version(but slower):
> + * {
> + *	struct page *page[1];
> + *
> + *	if (__get_user_pages_fast(addr, 1, 1, page) == 1) {
> + *		*writable = 1;
> + *		return page[0];
> + *	}
> + *	if (__get_user_pages_fast(addr, 1, 0, page) == 1) {
> + *		*writable = 0;
> + *		return page[0];
> + *	}
> + *	return NULL;
> + * }
> + */
> +struct page *get_user_page_and_protection(unsigned long addr, int *writable)
> +{
>
>    

<snip lots of code>

> +}
> +EXPORT_SYMBOL_GPL(get_user_page_and_protection);
> +
>    

This a duplication of __get_user_pages_fast(), no?

I think a better way is to have a __get_user_pages_ptes(..., struct page 
*pages, pte_t *ptes, ...), and write __get_user_pages_fast() in terms of 
that.  There's lots of useful info from the pte we can use:

- writeable bit (used here)
- page size (we can stick it in some user bits, speeds up 
host_mapping_level())
- dirty bit (if set, we can set it for speculative sptes and save the 
CPU a RMW)
- page attribute (for device assignment, when snooping is not available)

Nick, is __get_user_pages_ptes() fine with you?

-- 
error compiling committee.c: too many arguments to function

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