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

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Date:   Fri, 8 Dec 2017 08:37:43 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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 11:39 AM, Kirill A. Shutemov wrote:
> On Mon, Dec 04, 2017 at 04:34:45PM +0000, 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:
> 
> sme_pgtable_calc() also looks unnecessary complex.

I have no objections to improving this (although I just submitted a patch
that modifies this area, so this will have to be updated now).

> 
> Any objections on this:
> 
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 65e0d68f863f..59b7d7ba9b37 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -548,8 +548,7 @@ static void __init *sme_populate_pgd(pgd_t *pgd_base, void *pgtable_area,
>   
>   static unsigned long __init sme_pgtable_calc(unsigned long len)
>   {
> -	unsigned long p4d_size, pud_size, pmd_size;
> -	unsigned long total;
> +	unsigned long entries, tables;
>   
>   	/*
>   	 * Perform a relatively simplistic calculation of the pagetable
> @@ -559,41 +558,25 @@ static unsigned long __init sme_pgtable_calc(unsigned long len)
>   	 * mappings. Incrementing the count for each covers the case where
>   	 * the addresses cross entries.
>   	 */
> -	if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
> -		p4d_size = (ALIGN(len, PGDIR_SIZE) / PGDIR_SIZE) + 1;
> -		p4d_size *= sizeof(p4d_t) * PTRS_PER_P4D;
> -		pud_size = (ALIGN(len, P4D_SIZE) / P4D_SIZE) + 1;
> -		pud_size *= sizeof(pud_t) * PTRS_PER_PUD;
> -	} else {
> -		p4d_size = 0;
> -		pud_size = (ALIGN(len, PGDIR_SIZE) / PGDIR_SIZE) + 1;
> -		pud_size *= sizeof(pud_t) * PTRS_PER_PUD;
> -	}
> -	pmd_size = (ALIGN(len, PUD_SIZE) / PUD_SIZE) + 1;
> -	pmd_size *= sizeof(pmd_t) * PTRS_PER_PMD;
>   
> -	total = p4d_size + pud_size + pmd_size;
> +        entries = (DIV_ROUND_UP(len, PGDIR_SIZE) + 1) * PAGE_SIZE;

I stayed away from using PAGE_SIZE directly because other areas/files used
the sizeof() * PTRS_PER_ and I was trying to be consistent. Not that the
size of a page table is ever likely to change, but maybe defining a macro
(similar to the one in mm/pgtable.c) would be best rather than using
PAGE_SIZE directly.  Not required, just my opinion.

> +        if (PTRS_PER_P4D > 1)
> +                entries += (DIV_ROUND_UP(len, P4D_SIZE) + 1) * PAGE_SIZE;
> +        entries += (DIV_ROUND_UP(len, PUD_SIZE) + 1) * PAGE_SIZE;
> +        entries += (DIV_ROUND_UP(len, PMD_SIZE) + 1) * PAGE_SIZE;
>   
>   	/*
>   	 * Now calculate the added pagetable structures needed to populate
>   	 * the new pagetables.
>   	 */
> -	if (IS_ENABLED(CONFIG_X86_5LEVEL)) {
> -		p4d_size = ALIGN(total, PGDIR_SIZE) / PGDIR_SIZE;
> -		p4d_size *= sizeof(p4d_t) * PTRS_PER_P4D;
> -		pud_size = ALIGN(total, P4D_SIZE) / P4D_SIZE;
> -		pud_size *= sizeof(pud_t) * PTRS_PER_PUD;
> -	} else {
> -		p4d_size = 0;
> -		pud_size = ALIGN(total, PGDIR_SIZE) / PGDIR_SIZE;
> -		pud_size *= sizeof(pud_t) * PTRS_PER_PUD;
> -	}
> -	pmd_size = ALIGN(total, PUD_SIZE) / PUD_SIZE;
> -	pmd_size *= sizeof(pmd_t) * PTRS_PER_PMD;
>   
> -	total += p4d_size + pud_size + pmd_size;
> +        tables = DIV_ROUND_UP(entries, PGDIR_SIZE) * PAGE_SIZE;
> +        if (PTRS_PER_P4D > 1)
> +                tables += DIV_ROUND_UP(entries, P4D_SIZE) * PAGE_SIZE;
> +        tables += DIV_ROUND_UP(entries, PUD_SIZE) * PAGE_SIZE;
> +        tables += DIV_ROUND_UP(entries, PMD_SIZE) * PAGE_SIZE;
>   
> -	return total;
> +	return entries + tables;
>   }

It all looks reasonable, but I won't be able to test for the next few
days, though.

Thanks,
Tom

>   
>   void __init sme_encrypt_kernel(void)
>