linux-kernel - Re: [PATCH RFC v8 12/56] x86/sev: Add RMP entry lookup helpers

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Date:   Fri, 3 Mar 2023 16:28:39 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     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,
        bp@...en8.de, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH RFC v8 12/56] x86/sev: Add RMP entry lookup helpers

On 2/20/23 19:38, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> The snp_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 AMD PPR, see
> https://bugzilla.kernel.org/attachment.cgi?id=296015.
> 
> Co-developed-by: Ashish Kalra <ashish.kalra@....com>
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---

> +/*
> + * Return 1 if the RMP entry is assigned, 0 if it exists but is not assigned,
> + * and -errno if there is no corresponding RMP entry.
> + */

Hmm IMHO the kernel's idiomatic way is to return 0 on "success" and I'd
assume the more intuitive expectation of success here if the entry is
assigned? The various callers seem to differ though so I guess it depends on
context. Some however don't distinguish their "failure" from an ERR and
maybe they should, at least for the purposes of the various printks?

> +int snp_lookup_rmpentry(u64 pfn, int *level)
> +{
> +	struct rmpentry *e;
> +
> +	e = __snp_lookup_rmpentry(pfn, level);
> +	if (IS_ERR(e))
> +		return PTR_ERR(e);
> +
> +	return !!rmpentry_assigned(e);
> +}
> +EXPORT_SYMBOL_GPL(snp_lookup_rmpentry);