Re: [PATCH v17] x86/split_lock: Enable split lock detection by kernel

[Sean Christopherson]

On Sun, Jan 26, 2020 at 12:05:35PM -0800, Luck, Tony wrote:
> +/*
> + * Locking is not required at the moment because only bit 29 of this
> + * MSR is implemented and locking would not prevent that the operation
> + * of one thread is immediately undone by the sibling thread.
> + * Use the "safe" versions of rdmsr/wrmsr here because although code
> + * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should
> + * exist, there may be glitches in virtualization that leave a guest
> + * with an incorrect view of real h/w capabilities.
> + */
> +static bool __sld_msr_set(bool on)
> +{
> +	u64 test_ctrl_val;
> +
> +	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
> +		return false;

How about caching the MSR value on a per-{cpu/core} basis at boot to avoid
the RDMSR when switching to/from from a misbehaving tasks?  E.g. to avoid
penalizing well-behaved tasks any more than necessary.

We've likely got bigger issues if MSR_TEST_CTL is being written by BIOS
at runtime, even if the writes were limited to synchronous calls from the
kernel.

Probably makes sense to split the MSR's init sequence and runtime sequence,
e.g. to also use an unsafe wrmsrl() at runtime so that an unexpected #GP
generates a WARN.

> +
> +	if (on)
> +		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +	else
> +		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +
> +	return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val);
> +}