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

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Date:   Thu, 3 Mar 2022 09:09:37 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v11 22/45] x86/sev: Use SEV-SNP AP creation to start
 secondary CPUs

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);