linux-kernel - Re: [PATCH] x86/mm: Rewrite sme_populate

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Date:   Fri, 8 Dec 2017 08:43:55 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Borislav Petkov <bp@...e.de>,
        Brijesh Singh <brijesh.singh@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Rewrite sme_populate_pgd() in a more sensible way

On 12/4/2017 12:50 PM, Kirill A. Shutemov wrote:
> On Mon, Dec 04, 2017 at 12:33:01PM -0600, Tom Lendacky wrote:
>> On 12/4/2017 10:34 AM, Kirill A. Shutemov wrote:
>>> On Mon, Dec 04, 2017 at 04:00:26PM +0000, Tom Lendacky wrote:
>>>> On 12/4/2017 8:57 AM, Kirill A. Shutemov wrote:
>>>>> On Mon, Dec 04, 2017 at 08:19:11AM -0600, Tom Lendacky wrote:
>>>>>> On 12/4/2017 5:23 AM, Kirill A. Shutemov wrote:
>>>>>>> sme_populate_pgd() open-codes a lot of things that are not needed to be
>>>>>>> open-coded.
>>>>>>>
>>>>>>> Let's rewrite it in a more stream-lined way.
>>>>>>>
>>>>>>> This would also buy us boot-time switching between support between
>>>>>>> paging modes, when rest of the pieces will be upstream.
>>>>>>
>>>>>> Hi Kirill,
>>>>>>
>>>>>> Unfortunately, some of these can't be changed.  The use of p4d_offset(),
>>>>>> pud_offset(), etc., use non-identity mapped virtual addresses which cause
>>>>>> failures at this point of the boot process.
>>>>>
>>>>> Wat? Virtual address is virtual address. p?d_offset() doesn't care about
>>>>> what mapping you're using.
>>>>
>>>> Yes it does.  For example, pmd_offset() issues a pud_page_addr() call,
>>>> which does a __va() returning a non-identity mapped address (0xffff88...).
>>>> Only identity mapped virtual addresses have been setup at this point, so
>>>> the use of that virtual address panics the kernel.
>>>
>>> Stupid me. You are right.
>>>
>>> What about something like this:
>>>
>>> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
>>> index d9a9e9fc75dd..65e0d68f863f 100644
>>> --- a/arch/x86/mm/mem_encrypt.c
>>> +++ b/arch/x86/mm/mem_encrypt.c
>>> @@ -12,6 +12,23 @@
>>>    #define DISABLE_BRANCH_PROFILING
>>> +/*
>>> + * Since we're dealing with identity mappings, physical and virtual
>>> + * addresses are the same, so override these defines which are ultimately
>>> + * used by the headers in misc.h.
>>> + */
>>> +#define __pa(x)  ((unsigned long)(x))
>>> +#define __va(x)  ((void *)((unsigned long)(x)))
>>
>> No, you can't do this.  There are routines in this file that are called
>> after the kernel has switched to its standard virtual address map where
>> this definition of __va() will likely cause a failure.
> 
> Let's than split it up into separate compilation unit.
> 
>>> +/*
>>> + * Special hack: we have to be careful, because no indirections are
>>> + * allowed here, and paravirt_ops is a kind of one. As it will only run in
>>> + * baremetal anyway, we just keep it from happening. (This list needs to
>>> + * be extended when new paravirt and debugging variants are added.)
>>> + */
>>> +#undef CONFIG_PARAVIRT
>>> +#undef CONFIG_PARAVIRT_SPINLOCKS
>>
>> I'd really, really like to avoid doing something like this.
> 
> Any other proposals?
> 
> Current code is way too hairy and hard to modify.

I'd like to hear some other opinions, but if there are none, I don't see
an issue with splitting this up, I guess.  I just dislike doing some of
these hacks, but if it makes things cleaner and simpler to understand,
then I guess I can't really object.

Thanks,
Tom

>