Re: [PATCH v4 2/6] x86: document WC MTRR effects on PAT / non-PAT pages

Randy Dunlap
2015-04-30T22:01:22Z

On 04/29/15 14:44, Luis R. Rodriguez wrote:
> From: "Luis R. Rodriguez" <mcgrof@suse.com>
>

> ---
> Documentation/x86/mtrr.txt | 18 +++++++++++++++---
> Documentation/x86/pat.txt | 40 +++++++++++++++++++++++++++++++++++++++-
> arch/x86/kernel/cpu/mtrr/main.c | 3 +++
> 3 files changed, 57 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
> index cf08c9f..7e183e3 100644
> --- a/Documentation/x86/pat.txt
> +++ b/Documentation/x86/pat.txt
> @@ -102,7 +104,43 @@ wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc()
> as step 0 above and also track the usage of those pages and use set_memory_wb()
> before the page is freed to free pool.
>
> -
> +MTRR effects on PAT / non-PAT systems
> +-------------------------------------
> +
> +The following table provides the effects of using write-combining MTRRs when
> +using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally
> +mtrr_add() usage will be phased in favor of arch_phys_wc_add() which will
> +be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add()
> +is made should already have be ioremap'd with write-combining page attributes
> +or PAT entries, this can be done by using ioremap_wc() / or respective helpers.
> +Devices which combine areas of IO memory desired to remain uncachable with

I would spell it uncacheable. In kernel Documentation/, grep uncacheable finds
14 hits vs. 6 hits for uncachable. No big deal.

> +areas where write-combining is desirable and are restricted by the size
> +requirements of MTRRs should consider splitting up their IO memory space
> +cleanly with ioremap_uc() and ioremap_wc() followed by an arch_phys_wc_add()
> +encompassing both regions. Such use is nevertheless heavily discouraged as
> +the effective memory type is considered implementation defined. This strategy
> +should only be used as last resort on devices with size-contrained regions

size-constrained

> +where otherwise MTRR write-combining would not be effective.
> +
> +Note that you cannot use set_memory_wc() to override / whitelist IO remapped
> +memory space mapped with ioremap*() calls, set_memory_wc() can only be used
> +on RAM.
> +
> +----------------------------------------------------------------------
> +MTRR Non-PAT PAT Linux ioremap value Effective memory type
> +----------------------------------------------------------------------
> + Non-PAT | PAT
> + PAT
> + |PCD
> + ||PWT
> + |||
> +WC 000 WB _PAGE_CACHE_MODE_WB WC | WC
> +WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC
> +WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | WC
> +WC 011 UC _PAGE_CACHE_MODE_UC UC | UC
> +----------------------------------------------------------------------
> +
> +(*) denotes implementation defined and is discouraged
>
> Notes:
>
> diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> index ea5f363..12abdbe 100644
> --- a/arch/x86/kernel/cpu/mtrr/main.c
> +++ b/arch/x86/kernel/cpu/mtrr/main.c
> @@ -538,6 +538,9 @@ EXPORT_SYMBOL(mtrr_del);
> * attempts to add a WC MTRR covering size bytes starting at base and
> * logs an error if this fails.
> *
> + * The caller should expect to need to provide a power of two size on an

* The called should provide a power of two size on an equivalent
* power of two boundary.

> + * equivalent power of two boundary.
> + *
> * Drivers must store the return value to pass to mtrr_del_wc_if_needed,
> * but drivers should not try to interpret that return value.
> */
>

--
~Randy