Re: [PATCH v8 20/40] x86/sev: Use SEV-SNP AP creation to start secondary CPUs

[Vlastimil Babka]

On 12/31/21 16:36, Borislav Petkov wrote:
> On Fri, Dec 10, 2021 at 09:43:12AM -0600, Brijesh Singh wrote:
>> diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
>> index 123a96f7dff2..38c14601ae4a 100644
> 
>> +{
>> +	unsigned long pfn;
>> +	struct page *p;
>> +
>> +	/*
>> +	 * Allocate an SNP safe page to workaround the SNP erratum where
>> +	 * the CPU will incorrectly signal an RMP violation  #PF if a
>> +	 * hugepage (2mb or 1gb) collides with the RMP entry of VMSA page.
> 
> 		2MB or 1GB
> 
> Collides how? The 4K frame is inside the hugepage?
> 
>> +	 * The recommeded workaround is to not use the large page.
> 
> Unknown word [recommeded] in comment, suggestions:
>         ['recommended', 'recommend', 'recommitted', 'commended', 'commandeered']
> 
>> +	 *
>> +	 * Allocate one extra page, use a page which is not 2mb aligned
> 
> 2MB-aligned
> 
>> +	 * and free the other.
>> +	 */
>> +	p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
>> +	if (!p)
>> +		return NULL;
>> +
>> +	split_page(p, 1);
>> +
>> +	pfn = page_to_pfn(p);
>> +	if (IS_ALIGNED(__pfn_to_phys(pfn), PMD_SIZE)) {
>> +		pfn++;
>> +		__free_page(p);
>> +	} else {
>> +		__free_page(pfn_to_page(pfn + 1));
>> +	}
> 
> AFAICT, this is doing all this stuff so that you can make sure you get a
> non-2M-aligned page. I wonder if there's a way to simply ask mm to give
> you such page directly.
> 
> vbabka?

AFAIK, not, as this is a very unusual constraint. Unless there are more
places that need it, should be fine to solve it like above. Maybe just also
be optimistic and try a plain order-0 first and only if it has the undesired
alignment (which should happen only once per 512 allocations), fallback to
the above?