linux-kernel - Re: [PATCH v10 23/50] KVM: SEV: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

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Date: Mon, 18 Dec 2023 15:57:52 +0100
From: Borislav Petkov <bp@...en8.de>
To: Michael Roth <michael.roth@....com>
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org,
	linux-crypto@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
	ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
	vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
	rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
	tony.luck@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
	alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
	nikunj.dadhania@....com, pankaj.gupta@....com,
	liam.merwick@...cle.com, zhi.a.wang@...el.com,
	Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 23/50] KVM: SEV: Make AVIC backing, VMSA and VMCB
 memory allocation SNP safe


Just typos:

On Mon, Oct 16, 2023 at 08:27:52AM -0500, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> Implement a workaround for an SNP erratum where the CPU will incorrectly
> signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
> RMP entry of a VMCB, VMSA or AVIC backing page.
> 
> When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
> backing pages as "in-use" via a reserved bit in the corresponding RMP
> entry after a successful VMRUN. This is done for _all_ VMs, not just
> SNP-Active VMs.
> 
> If the hypervisor accesses an in-use page through a writable
> translation, the CPU will throw an RMP violation #PF. On early SNP
> hardware, if an in-use page is 2mb aligned and software accesses any
> part of the associated 2mb region with a hupage, the CPU will

"hugepage"

> incorrectly treat the entire 2mb region as in-use and signal a spurious
> RMP violation #PF.
> 
> The recommended is to not use the hugepage for the VMCB, VMSA or

s/recommended/recommendation/
s/the hugepage/a hugepage/

> AVIC backing page for similar reasons. Add a generic allocator that will
> ensure that the page returns is not hugepage (2mb or 1gb) and is safe to

"... the page returned is not a hugepage..."

...

> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu)
> +{
> +	unsigned long pfn;
> +	struct page *p;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +
> +	/*
> +	 * 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 VMCB, VMSA
> +	 * or AVIC backing page. The recommeded workaround is to not use the

"recommended"

> +	 * hugepage.
> +	 *
> +	 * 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, PTRS_PER_PMD))
> +		__free_page(p++);
> +	else
> +		__free_page(p + 1);
> +
> +	return p;
> +}

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette