linux-kernel - Re: [PATCHv2 2/5] x86/tme: Detect if TME and MKTME is activated by BIOS

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Date:   Wed, 7 Feb 2018 11:02:26 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/5] x86/tme: Detect if TME and MKTME is activated by
 BIOS

On 02/07/2018 04:59 AM, Kirill A. Shutemov wrote:
> IA32_TME_ACTIVATE MSR (0x982) can be used to check if BIOS has enabled
> TME and MKTME. It includes which encryption policy/algorithm is selected
> for TME or available for MKTME. For MKTME, the MSR also enumerates how
> many KeyIDs are available.

The hacking of the phys_addr_bits is a pretty important part of this.
Are you sure it's not worth calling out in the description?

> +#define MSR_IA32_TME_ACTIVATE		0x982
> +
> +#define TME_ACTIVATE_LOCKED(x)		(x & 0x1)
> +#define TME_ACTIVATE_ENABLED(x)		(x & 0x2)
> +
> +#define TME_ACTIVATE_POLICY(x)		((x >> 4) & 0xf)	/* Bits 7:4 */
> +#define TME_ACTIVATE_POLICY_AES_XTS_128	0
> +
> +#define TME_ACTIVATE_KEYID_BITS(x)	((x >> 32) & 0xf)	/* Bits 35:32 */
> +
> +#define TME_ACTIVATE_CRYPTO_ALGS(x)	((x >> 48) & 0xffff)	/* Bits 63:48 */
> +#define TME_ACTIVATE_CRYPTO_AES_XTS_128	1
> +
> +#define MKTME_ENABLED		0
> +#define MKTME_DISABLED		1
> +#define MKTME_UNINITIALIZED	2

The indentation there looks a bit wonky.  Might want to double-check it.

Also, can you clearly spell out which of these things are software
constructs vs. hardware ones?  MKTME_* look like software constructs.

> +static int mktme_status = MKTME_UNINITIALIZED;
> +
> +static void detect_keyid_bits(struct cpuinfo_x86 *c, u64 tme_activate)
> +{
> +	int keyid_bits = 0, nr_keyids = 0;
> +
> +	keyid_bits = TME_ACTIVATE_KEYID_BITS(tme_activate);
> +	nr_keyids = (1UL << keyid_bits) - 1;
> +	if (nr_keyids) {
> +		pr_info_once("x86/mktme: enabled by BIOS\n");
> +		pr_info_once("x86/mktme: %d KeyIDs available\n", nr_keyids);
> +	} else {
> +		pr_info_once("x86/mktme: disabled by BIOS\n");
> +	}

Just curious, but how do you know that this indicates the BIOS disabling
MKTME?

> +	if (mktme_status == MKTME_UNINITIALIZED) {
> +		/* MKTME is usable */
> +		mktme_status = MKTME_ENABLED;
> +	}

To me, it's a little bit odd that we "enable" MKTME down in the keyid
detection code.  I wonder if you could just return the resulting number
of keyids and then actually do the mktme_status munging in one place
(detect_tme()).

> +	/*
> +	 * Exclude KeyID bits from physical address bits.
> +	 *
> +	 * We have to do this even if we are not going to use KeyID bits
> +	 * ourself. VM guests still have to know that these bits are not usable
> +	 * for physical address.
> +	 */
> +	c->x86_phys_bits -= keyid_bits;
> +}

How do we tell guests about this?  kvm_set_mmio_spte_mask()?

> +static void detect_tme(struct cpuinfo_x86 *c)
> +{
> +	u64 tme_activate, tme_policy, tme_crypto_algs;
> +	static u64 tme_activate_cpu0 = 0;
> +
> +	rdmsrl(MSR_IA32_TME_ACTIVATE, tme_activate);
> +
> +	if (mktme_status != MKTME_UNINITIALIZED) {
> +		if (tme_activate != tme_activate_cpu0) {
> +			/* Broken BIOS? */
> +			pr_err_once("x86/tme: configuation is inconsistent between CPUs\n");
> +			pr_err_once("x86/tme: MKTME is not usable\n");
> +			mktme_status = MKTME_DISABLED;
> +
> +			/* Proceed. We may need to exclude bits from x86_phys_bits. */
> +		}
> +	} else {
> +		tme_activate_cpu0 = tme_activate;
> +	}
> +
> +	if (!TME_ACTIVATE_LOCKED(tme_activate) || !TME_ACTIVATE_ENABLED(tme_activate)) {
> +		pr_info_once("x86/tme: not enabled by BIOS\n");
> +		mktme_status = MKTME_DISABLED;
> +		return;
> +	}
> +
> +	if (mktme_status != MKTME_UNINITIALIZED)
> +		return detect_keyid_bits(c, tme_activate);

Returning the result of a void function is a bit odd-looking.  Would it
look nicer to just have a label and some gotos to the detection?

> +	pr_info("x86/tme: enabled by BIOS\n");
> +
> +	tme_policy = TME_ACTIVATE_POLICY(tme_activate);
> +	if (tme_policy != TME_ACTIVATE_POLICY_AES_XTS_128)
> +		pr_warn("x86/tme: Unknown policy is active: %#llx\n", tme_policy);
> +
> +	tme_crypto_algs = TME_ACTIVATE_CRYPTO_ALGS(tme_activate);
> +	if (!(tme_crypto_algs & TME_ACTIVATE_CRYPTO_AES_XTS_128)) {
> +		pr_err("x86/mktme: No known encryption algorithm is supported: %#llx\n",
> +				tme_crypto_algs);
> +		mktme_status = MKTME_DISABLED;
> +	}
> +
> +	detect_keyid_bits(c, tme_activate);
> +}

I noticed that this code is not optional, other than by disabling
CPU_SUP_INTEL.  Was that intentional?  What were your thoughts behind that?