Re: [PATCH v1 1/1] x86/cpu: Fix boot on Intel Quark X1000

[Dave Hansen]

On 5/16/24 10:39, Andy Shevchenko wrote:
> The initial change to set x86_virt_bits to the correct value straight
> away broke boot on Intel Quark X1000 CPUs (which are family 5, model 9,
> stepping 0)

Do you know what _actually_ broke?  Like was there a crash somewhere?

> With deeper investigation it appears that the Quark doesn't have
> the bit 19 set in 0x01 CPUID leaf, which means it doesn't provide
> any clflush instructions and hence the cache alignment is set to 0.
> The actual cache line size is 16 bytes, hence we may set the alignment
> accordingly. At the same time the physical and virtual address bits
> are retrieved via 0x80000008 CPUID leaf.

This seems to be saying that ->x86_clflush_size must come from CPUID.
But there _are_ CPUID-independent defaults set in identify_cpu().  How
do those fit in?

> Note, we don't really care about the value of x86_clflush_size as it
> is either used with a proper check for the instruction to be present,
> or, like in PCI case, it assumes 32 bytes for all supported 32-bit CPUs
> that have actually smaller cache line sizes and don't advertise it.

Are you trying to say that having ->x86_clflush_size==0 is not fatal
while having ->x86_cache_alignment==0 _is_ fatal?

> The commit fbf6449f84bf ("x86/sev-es: Set x86_virt_bits to the correct
> value straight away, instead of a two-phase approach") basically
> revealed the issue that has been present from day 1 of introducing
> the Quark support.

How did it do that, exactly?  It's still not crystal clear.

> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index be30d7fa2e66..2bffae158dd5 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -321,6 +321,15 @@ static void early_init_intel(struct cpuinfo_x86 *c)
>  #ifdef CONFIG_X86_64
>  	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
>  #else
> +	/*
> +	 * The Quark doesn't have bit 19 set in 0x01 CPUID leaf, which means
> +	 * it doesn't provide any clflush instructions and hence the cache
> +	 * alignment is set to 0. The actual cache line size is 16 bytes,
> +	 * hence set the alignment accordingly. At the same time the physical
> +	 * and virtual address bits are retrieved via 0x80000008 CPUID leaf.
> +	 */
> +	if (c->x86 == 5 && c->x86_model == 9)
> +		c->x86_cache_alignment = 16;

What are the odds that another CPU has this issue?  I'm thinking we
should just set a default in addition to hacking around this for Quark.