Re: [PATCH v4] xen/x86: On x2APIC mode, derive LDR from APIC ID

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

A few minor grammar notes.

"x86/vlapic: In x2APIC ..."


On 23/11/2023 5:30 pm, 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)

Missing full stop.  But I'd also phrase this explicitly as "violating
the spec", rather than "problematic".

Intel is crystal clear on the matter:

  Logical x2APIC ID = [(x2APIC ID[19:4] « 16) | (1 « x2APIC ID[3:0])]

and Xen isn't implementing this.

> This patch fixes the implementation so we follow the rules in the x2APIC
> spec(s) and covers migrations from broken hypervisors, so LDRs are
> preserved even for hotppluggable CPUs and across APIC resets.

"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."

Hotpluggable isn't relevant.  We have the state, and it's either as it
was before, or it's fixed.


>
> While touching that area, I removed 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>
> ---
> v4:
>   * Removed "See <function>()" part of comment in set_x2apic_id()
>   * Removed _with_ in field name
>   * Trimmed down comments further
>   * Rephrased the Xen versions in the comments so it's implied not every
>     Xen 4.X is affected (as they won't be after this patch is backported)
>   * Replace Xen 4.18 reference with date+4.19 dev window
> ---
>  xen/arch/x86/hvm/vlapic.c             | 66 ++++++++++++++++++---------
>  xen/arch/x86/include/asm/hvm/domain.h |  3 ++
>  2 files changed, 47 insertions(+), 22 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
> index 5cb87f8649..cd4701c5a2 100644
> --- a/xen/arch/x86/hvm/vlapic.c
> +++ b/xen/arch/x86/hvm/vlapic.c
> @@ -1061,13 +1061,26 @@ static const struct hvm_mmio_ops vlapic_mmio_ops = {
>      .write = vlapic_mmio_write,
>  };
>  
> +static uint32_t x2apic_ldr_from_id(uint32_t id)
> +{
> +    return ((id & ~0xf) << 12) | (1 << (id & 0xf));
> +}
> +
>  static void set_x2apic_id(struct vlapic *vlapic)
>  {

const struct vcpu *v = vlapic_vcpu(vlapic);

seeing as you've got the expression used twice already.

With that, the following logic is shorter too; you can get away with
dropping the vcpu_id variable as v->vcpu_id is the more common form to
use in Xen.


> -    u32 id = vlapic_vcpu(vlapic)->vcpu_id;
> -    u32 ldr = ((id & ~0xf) << 12) | (1 << (id & 0xf));
> +    uint32_t vcpu_id = vlapic_vcpu(vlapic)->vcpu_id;
> +    uint32_t apic_id = vcpu_id * 2;

/* TODO: Fix topology */ as a suffix here.

Just to make it clear that we're aware that this *2 logic is faulty, but
it needs to remain like this in the short term.

> +    uint32_t apic_ldr = x2apic_ldr_from_id(apic_id);
> +
> +    /*
> +     * Workaround for migrated domains to derive LDRs as the source host
> +     * would've.
> +     */
> +    if ( vlapic_domain(vlapic)->arch.hvm.bug_x2apic_ldr_vcpu_id )
> +        apic_ldr = x2apic_ldr_from_id(vcpu_id);
>  
> -    vlapic_set_reg(vlapic, APIC_ID, id * 2);
> -    vlapic_set_reg(vlapic, APIC_LDR, ldr);
> +    vlapic_set_reg(vlapic, APIC_ID, apic_id);
> +    vlapic_set_reg(vlapic, APIC_LDR, apic_ldr);
>  }
>  
>  int guest_wrmsr_apic_base(struct vcpu *v, uint64_t val)
> @@ -1498,27 +1511,36 @@ static int cf_check lapic_save_regs(struct vcpu *v, 
> hvm_domain_context_t *h)
>   */
>  static void lapic_load_fixup(struct vlapic *vlapic)
>  {

Again, const struct vcpu *v = vlapic_vcpu(vlapic); here helps legibility.

> -    uint32_t id = vlapic->loaded.id;
> +    uint32_t good_ldr = x2apic_ldr_from_id(vlapic->loaded.id);
>  
> -    if ( vlapic_x2apic_mode(vlapic) && id && vlapic->loaded.ldr == 1 )
> -    {
> +    /* Skip fixups on xAPIC mode, or if the x2APIC LDR is already correct */
> +    if ( !vlapic_x2apic_mode(vlapic) ||
> +         (vlapic->loaded.ldr == good_ldr) )
> +        return;
> +
> +    if ( vlapic->loaded.ldr == 1 )
> +       /*
> +        * Xen <= 4.4 may have a bug by which all the APICs configured in
> +        * x2APIC mode got LDR = 1. We can't leave it as-is because it
> +        * assigned the same LDR to every CPU.  We'll fix fix the bug now
> +        * and assign an LDR value consistent with the APIC ID.
> +        */
> +        set_x2apic_id(vlapic);
> +    else if ( vlapic->loaded.ldr ==

I know these are single statement if's, and the coding style permits
them without braces, but the comment makes it visually awkward to follow.

This is an example where braces help readability generally, but also (as
it happens) help make a more readable diff.


> +              x2apic_ldr_from_id(vlapic_vcpu(vlapic)->vcpu_id) )
>          /*
> -         * This is optional: ID != 0 contradicts LDR == 1. It's being added
> -         * to aid in eventual debugging of issues arising from the fixup done
> -         * here, but can be dropped as soon as it is found to conflict with
> -         * other (future) changes.
> +         * Migrations from Xen 4.4 to date (4.19 dev window, Nov 2023) may
> +         * show this bug.

"may have had LDR derived from the vCPU_ID, not the APIC_ID."

Better to clearly state what the bug is.

>  We must preserve LDRs so new vCPUs use consistent
> +         * derivations and existing guests, which may have already read the
> +         * LDR at the source host, aren't surprised when interrupts stop
> +         * working the way they did at the other end.
>           */
> -        if ( GET_xAPIC_ID(id) != vlapic_vcpu(vlapic)->vcpu_id * 2 ||
> -             id != SET_xAPIC_ID(GET_xAPIC_ID(id)) )
> -            printk(XENLOG_G_WARNING "%pv: bogus APIC ID %#x loaded\n",
> -                   vlapic_vcpu(vlapic), id);
> -        set_x2apic_id(vlapic);
> -    }
> -    else /* Undo an eventual earlier fixup. */
> -    {
> -        vlapic_set_reg(vlapic, APIC_ID, id);
> -        vlapic_set_reg(vlapic, APIC_LDR, vlapic->loaded.ldr);
> -    }
> +        vlapic_domain(vlapic)->arch.hvm.bug_x2apic_ldr_vcpu_id = true;
> +    else
> +        printk(XENLOG_G_WARNING
> +               "%pv: bogus x2APIC loaded id=%#x ldr=%#x (expected 
> ldr=%#x)\n",

%pv: bogus loaded x2APIC ID %#x, LDR %#x, expected LDR %#x\n

If you properly capitalise x2APIC, you should capitalise ID and LDR. 
The other changes are matter of taste, but make for a less cluttered log
message IMO.


This is a long list of minor niggles, but they're all style/comment
issues, and nothing to do with the logic itself.  I'm happy to fix them
all up on commit, and here is the result with them merged:

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

for you to glance over.

~Andrew