linux-kernel - Re: [RFC Part2 PATCH 02/30] x86/sev-snp: add RMP entry lookup helpers

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Date:   Thu, 15 Apr 2021 18:57:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        linux-crypto@...r.kernel.org, ak@...ux.intel.com,
        herbert@...dor.apana.org.au, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part2 PATCH 02/30] x86/sev-snp: add RMP entry lookup helpers

On Wed, Mar 24, 2021 at 12:04:08PM -0500, Brijesh Singh wrote:
> The lookup_page_in_rmptable() can be used by the host to read the RMP
> entry for a given page. The RMP entry format is documented in PPR
> section 2.1.5.2.

I see

Table 15-36. Fields of an RMP Entry

in the APM.

Which PPR do you mean? Also, you know where to put those documents,
right?

> +/* RMP table entry format (PPR section 2.1.5.2) */
> +struct __packed rmpentry {
> +	union {
> +		struct {
> +			uint64_t assigned:1;
> +			uint64_t pagesize:1;
> +			uint64_t immutable:1;
> +			uint64_t rsvd1:9;
> +			uint64_t gpa:39;
> +			uint64_t asid:10;
> +			uint64_t vmsa:1;
> +			uint64_t validated:1;
> +			uint64_t rsvd2:1;
> +		} info;
> +		uint64_t low;
> +	};
> +	uint64_t high;
> +};
> +
> +typedef struct rmpentry rmpentry_t;

Eww, a typedef. Why?

struct rmpentry is just fine.

> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 39461b9cb34e..06394b6d56b2 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -34,6 +34,8 @@
>  
>  #include "mm_internal.h"
>  

<--- Needs a comment here to explain the magic 0x4000 and the magic
shift by 8.

> +#define rmptable_page_offset(x)	(0x4000 + (((unsigned long) x) >> 8))
> +
>  /*
>   * Since SME related variables are set early in the boot process they must
>   * reside in the .data section so as not to be zeroed out when the .bss
> @@ -612,3 +614,33 @@ static int __init mem_encrypt_snp_init(void)
>   * SEV-SNP must be enabled across all CPUs, so make the initialization as a late initcall.
>   */
>  late_initcall(mem_encrypt_snp_init);
> +
> +rmpentry_t *lookup_page_in_rmptable(struct page *page, int *level)

snp_lookup_page_in_rmptable()

> +{
> +	unsigned long phys = page_to_pfn(page) << PAGE_SHIFT;
> +	rmpentry_t *entry, *large_entry;
> +	unsigned long vaddr;
> +
> +	if (!static_branch_unlikely(&snp_enable_key))
> +		return NULL;
> +
> +	vaddr = rmptable_start + rmptable_page_offset(phys);
> +	if (WARN_ON(vaddr > rmptable_end))

Do you really want to spew a warn on splat for each wrong vaddr? What
for?

> +		return NULL;
> +
> +	entry = (rmpentry_t *)vaddr;
> +
> +	/*
> +	 * Check if this page is covered by the large RMP entry. This is needed to get
> +	 * the page level used in the RMP entry.
> +	 *

No need for a new line in the comment and no need for the "e.g." thing
either.

Also, s/the large RMP entry/a large RMP entry/g.

> +	 * e.g. if the page is covered by the large RMP entry then page size is set in the
> +	 *       base RMP entry.
> +	 */
> +	vaddr = rmptable_start + rmptable_page_offset(phys & PMD_MASK);
> +	large_entry = (rmpentry_t *)vaddr;
> +	*level = rmpentry_pagesize(large_entry);
> +
> +	return entry;
> +}
> +EXPORT_SYMBOL_GPL(lookup_page_in_rmptable);

Exported for kvm?

-- 
Regards/Gruss,
    Boris.

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