[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] xen/x86: On x2APIC mode, derive LDR from APIC_ID
On Tue, Nov 14, 2023 at 11:14:22AM +0100, Jan Beulich wrote: > 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 You mean changing the LDR of a vCPU to the correct value on migrate? That feels like playing with fire. A migrated VM is presumably a VM that is running without issues (or it would have been rebooted). Letting it run as it did seems safer. I don't think vlapic_match_logical_addr() is affected. The LDR's are still unique in the bogus case so the matching ought to work. Problem would arise if the guest makes assumptions about APIC_ID and LDR relationships. Cheers, Alejandro
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