Re: [PATCH] xen/x86: On x2APIC mode, derive LDR from APIC_ID

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.11.2023 18:53, Roger Pau Monné wrote:
> On Mon, Nov 13, 2023 at 04:50:23PM +0000, Alejandro Vallejo wrote:
>> Both Intel and AMD manuals agree that on x2APIC mode, the APIC LDR and ID
>> registers are derivable from each other through a fixed formula.
>>
>> Xen uses that formula, but applies it to vCPU IDs (which are sequential)
>> rather than x2APIC_IDs (which are not, at the moment). As I understand it,
>> this is an attempt to tightly pack vCPUs into clusters so each cluster has
>> 16 vCPUs rather than 8, but this is problematic for OSs that might read the
>> x2APIC_ID and internally derive LDR (or the other way around)
> 
> I would replace the underscore from x2APIC ID with a space instead.
> 
> Seeing the commit that introduced the bogus LDR value, I'm not sure it
> was intentional,

Hard to reconstruct over 9 years later. It feels like Alejandro may be right
with his derivation.

> as previous Xen code had:
> 
> u32 id = vlapic_get_reg(vlapic, APIC_ID);
> u32 ldr = ((id & ~0xf) << 16) | (1 << (id & 0xf));
> vlapic_set_reg(vlapic, APIC_LDR, ldr);
> 
> Which was correct, as the LDR was derived from the APIC ID and not the
> vCPU ID.

Well, it gave the appearance of deriving from the APIC ID. Just that it was
missing GET_xAPIC_ID() around the vlapic_get_reg() (hence why LDR was
uniformly 1 on all CPUs).

>> This patch fixes the implementation so we follow the rules in the x2APIC
>> spec(s).
>>
>> While in the neighborhood, replace the u32 type with the standard uint32_t
> 
> Likely wants:
> 
> Fixes: f9e0cccf7b35 ('x86/HVM: fix ID handling of x2APIC emulation')

+1

>> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> 
> I do wonder whether we need to take any precautions with guests being
> able to trigger an APIC reset, and thus seeing a changed LDR register
> if the guest happens to be migrated from an older hypervisor version
> that doesn't have this fix.  IOW: I wonder whether Xen should keep the
> previous bogus LDR value across APIC resets for guests that have
> already seen it.

That earlier change deliberately fixed up any bogus values. I wonder
whether what you suggest will do more good or more harm than going
even farther and once again fixing up bad values in lapic_load_fixup().
After all LDR being wrong affects vlapic_match_logical_addr()'s outcome.
I think one of the two wants adding to the change, though.

Jan