linux-kernel - Re: [PATCH Part2 v6 06/49] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

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Message-ID: <d89c695e-7d45-160f-5e28-fee5ee576104@intel.com>
Date:   Wed, 22 Jun 2022 07:26:31 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ashish Kalra <Ashish.Kalra@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org
Cc:     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,
        michael.roth@....com, vbabka@...e.cz, 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
Subject: Re: [PATCH Part2 v6 06/49] x86/sev: Add helper functions for
 RMPUPDATE and PSMASH instruction

On 6/20/22 16:02, Ashish Kalra wrote:
> +int psmash(u64 pfn)
> +{
> +	unsigned long paddr = pfn << PAGE_SHIFT;
> +	int ret;
> +
> +	if (!pfn_valid(pfn))
> +		return -EINVAL;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +		return -ENXIO;
> +
> +	/* Binutils version 2.36 supports the PSMASH mnemonic. */
> +	asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> +		      : "=a"(ret)
> +		      : "a"(paddr)
> +		      : "memory", "cc");
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(psmash);

If a function gets an EXPORT_SYMBOL_GPL(), the least we can do is
reasonably document it.  We don't need full kerneldoc nonsense, but a
one-line about what this does would be quite helpful.  That goes for all
the functions here.

It would also be extremely helpful to have the changelog explain why
these functions are exported and how the exports will be used.

As a general rule, please push cpu_feature_enabled() checks as early as
you reasonably can.  They are *VERY* cheap and can even enable the
compiler to completely zap code like an #ifdef.

There also seem to be a lot of pfn_valid() checks in here that aren't
very well thought out.  For instance, there's a pfn_valid() check here:


+int rmp_make_shared(u64 pfn, enum pg_level level)
+{
+	struct rmpupdate val;
+
+	if (!pfn_valid(pfn))
+		return -EINVAL;
...
+	return rmpupdate(pfn, &val);
+}

and in rmpupdate():

+static int rmpupdate(u64 pfn, struct rmpupdate *val)
+{
+	unsigned long paddr = pfn << PAGE_SHIFT;
+	int ret;
+
+	if (!pfn_valid(pfn))
+		return -EINVAL;
...


This is (at best) wasteful.  Could it be refactored?