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

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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




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