Re: [PATCH v11 22/45] x86/sev: Use SEV-SNP AP creation to start secondary CPUs

[Brijesh Singh <brijesh.singh@amd.com>]

On 3/3/22 11:09, Dave Hansen wrote:
> On 2/24/22 08:56, Brijesh Singh wrote:
>> +	/*
>> +	 * Allocate VMSA page to work around the SNP erratum where the CPU will
>> +	 * incorrectly signal an RMP violation #PF if a large page (2MB or 1GB)
>> +	 * collides with the RMP entry of VMSA page. The recommended workaround
>> +	 * is to not use a large page.
>> +	 *
>> +	 * Allocate one extra page, use a page which is not 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));
>> +	}
>> +
>> +	return page_address(pfn_to_page(pfn));
>> +}
> 
> This can be simplified.  There's no need for all the sill pfn_to_page()
> conversions or even an alignment check.  The second page (page[1]) of an
> order-1 page is never 2M/1G aligned.  Just use that:
> 
> 	// Alloc an 8k page which is also 8k-aligned:
> 	p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> 	if (!p)
> 		return NULL;
> 
> 	split_page(p, 1);
> 
> 	// Free the first 4k.  This page _may_
> 	// be 2M/1G aligned and can not be used:
> 	__free_page(p);
> 	
> 	// Return the unaligned page:
> 	return page_address(p+1);
> 

Ack, I am good with using your version.

-Brijesh