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 12:01:57 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.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:     brijesh.singh@....com, 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 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