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

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Date:   Wed, 22 Jun 2022 18:04:40 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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Subject: RE: [PATCH Part2 v6 06/49] x86/sev: Add helper functions for
 RMPUPDATE and PSMASH instruction

[AMD Official Use Only - General]

>> +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.

I will add basic descriptions for all these exported functions.

Thanks,
Ashish

>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?