Re: [PATCH v5] xen/x86: On x2APIC mode, derive LDR from APIC ID

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

On 27/11/2023 13:46, 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 a spec violation.
> 
> This patch fixes the implementation so we follow the x2APIC spec for new
> VMs, while preserving the behaviour (buggy or fixed) for migrated-in VMs.
> 
> While touching that area, remove the existing printk statement in
> vlapic_load_fixup() (as the checks it performed didn't make sense in x2APIC
> mode and wouldn't affect the outcome) and put another printk as an else
> branch so we get warnings trying to load nonsensical LDR values we don't
> know about.
> 
> Fixes: f9e0cccf7b35 ("x86/HVM: fix ID handling of x2APIC emulation")
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

While no R-by from Andrew was in the mailing list, it was in the xenbits
patch, of which this is a direct copy except for minor delta suggested
by Jan in lapic_load_fixup()

> https://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=commitdiff;h=953dcb0317d20959ffee14e404595cfbb66c607a#patch1

Cheers,
Alejandro