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    SubjectRe: [PATCH v5 00/16] x86: make PAT and MTRR independent from each other
    On 07.11.22 20:25, Borislav Petkov wrote:
    > On Thu, Nov 03, 2022 at 05:15:52PM +0100, Borislav Petkov wrote:
    >> Lemme try to find a smaller box which shows that too - that one is a
    >> pain to bisect on.
    >
    > Ok, couldn't find a smaller one (or maybe it had to be a big one to
    > tickle this out).
    >
    > So I think it is the parallel setup thing:
    >
    > x86/mtrr: Do MTRR/PAT setup on all secondary CPUs in parallel
    >
    > Note that before it, it would do the configuration sequentially on each
    > CPU:
    >
    > [ 0.759239] MTRR: prepare_set: CPU83, MSR_MTRRdefType: 0x0, read: (0xc00:0)
    > [ 0.759239] MTRR: set_mtrr_state: CPU83, mtrr_deftype_lo: 0xc00, mtrr_state.def_type: 0, mtrr_state.enabled: 3
    > [ 0.760794] MTRR: post_set: CPU83, MSR_MTRRdefType will write: (0xc00:0)
    > [ 0.761151] MTRR: prepare_set: CPU70, MSR_MTRRdefType: 0x0, read: (0xc00:0)
    > [ 0.761151] MTRR: set_mtrr_state: CPU70, mtrr_deftype_lo: 0xc00, mtrr_state.def_type: 0, mtrr_state.enabled: 3
    > [ 0.761151] MTRR: post_set: CPU70, MSR_MTRRdefType will write: (0xc00:0)
    > ...
    >
    > and so on.
    >
    > Now, it would do it all in parallel:
    >
    > [ 0.762006] MTRR: mtrr_disable: CPU70, MSR_MTRRdefType: 0x0, read: (0xc00:0)
    > [ 0.761916] MTRR: mtrr_disable: CPU18, MSR_MTRRdefType: 0x0, read: (0xc00:0)
    > [ 0.761808] MTRR: mtrr_disable: CPU82, MSR_MTRRdefType: 0x0, read: (0xc00:0)
    > [ 0.762593] MTRR: mtrr_disable: CPU6, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > ^^^^^^
    >
    > Note that last thing. That comes from (with debug output added):
    >
    > void mtrr_disable(struct cache_state *state)
    > {
    > unsigned int cpu = smp_processor_id();
    > u64 msrval;
    >
    > /* Save MTRR state */
    > rdmsr(MSR_MTRRdefType, state->mtrr_deftype_lo, state->mtrr_deftype_hi);
    >
    > /* Disable MTRRs, and set the default type to uncached */
    > mtrr_wrmsr(MSR_MTRRdefType, state->mtrr_deftype_lo & ~0xcff,
    > state->mtrr_deftype_hi);
    >
    > rdmsrl(MSR_MTRRdefType, msrval);
    >
    > pr_info("%s: CPU%d, MSR_MTRRdefType: 0x%llx, read: (0x%x:%x)\n",
    > __func__, cpu, msrval, state->mtrr_deftype_lo, state->mtrr_deftype_hi);
    > }
    >
    > The "read: (0x0:0)" basically says that
    >
    > state->mtrr_deftype_lo, state->mtrr_deftype_hi
    >
    > are both 0 already. BUT(!), they should NOT be. The low piece is 0xc00 on most
    > cores except a handful and it means that MTRRs and Fixed Range are
    > enabled. In total, they're these cores here:
    >
    > [ 0.762593] MTRR: mtrr_disable: CPU6, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762247] MTRR: mtrr_disable: CPU26, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762685] MTRR: mtrr_disable: CPU68, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762725] MTRR: mtrr_disable: CPU17, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762685] MTRR: mtrr_disable: CPU69, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762800] MTRR: mtrr_disable: CPU1, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762734] MTRR: mtrr_disable: CPU13, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762720] MTRR: mtrr_disable: CPU24, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762696] MTRR: mtrr_disable: CPU66, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762716] MTRR: mtrr_disable: CPU48, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762693] MTRR: mtrr_disable: CPU57, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762519] MTRR: mtrr_disable: CPU87, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762532] MTRR: mtrr_disable: CPU58, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762755] MTRR: mtrr_disable: CPU32, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762693] MTRR: mtrr_disable: CPU52, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762861] MTRR: mtrr_disable: CPU0, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762724] MTRR: mtrr_disable: CPU21, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762640] MTRR: mtrr_disable: CPU15, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762615] MTRR: mtrr_disable: CPU50, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762741] MTRR: mtrr_disable: CPU40, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762738] MTRR: mtrr_disable: CPU37, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762716] MTRR: mtrr_disable: CPU25, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762512] MTRR: mtrr_disable: CPU59, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762721] MTRR: mtrr_disable: CPU45, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762682] MTRR: mtrr_disable: CPU56, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762583] MTRR: mtrr_disable: CPU124, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762751] MTRR: mtrr_disable: CPU12, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762741] MTRR: mtrr_disable: CPU9, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762575] MTRR: mtrr_disable: CPU51, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762632] MTRR: mtrr_disable: CPU100, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762688] MTRR: mtrr_disable: CPU61, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762610] MTRR: mtrr_disable: CPU105, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762721] MTRR: mtrr_disable: CPU20, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.762583] MTRR: mtrr_disable: CPU47, MSR_MTRRdefType: 0x0, read: (0x0:0)
    >
    > Now, if I add MFENCEs around those RDMSRs:
    >
    > void mtrr_disable(struct cache_state *state)
    > {
    > unsigned int cpu = smp_processor_id();
    > u64 msrval;
    >
    > /* Save MTRR state */
    > rdmsr(MSR_MTRRdefType, state->mtrr_deftype_lo, state->mtrr_deftype_hi);
    >
    > __mb();
    >
    > /* Disable MTRRs, and set the default type to uncached */
    > mtrr_wrmsr(MSR_MTRRdefType, state->mtrr_deftype_lo & ~0xcff,
    > state->mtrr_deftype_hi);
    >
    > __mb();
    >
    > rdmsrl(MSR_MTRRdefType, msrval);
    >
    > pr_info("%s: CPU%d, MSR_MTRRdefType: 0x%llx, read: (0x%x:%x)\n",
    > __func__, cpu, msrval, state->mtrr_deftype_lo, state->mtrr_deftype_hi);
    >
    > __mb();
    > }
    >
    > the amount of cores becomes less:

    Probably not because of the fencing, but because of the different timing.

    >
    > [ 0.765260] MTRR: mtrr_disable: CPU6, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765462] MTRR: mtrr_disable: CPU5, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765242] MTRR: mtrr_disable: CPU22, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765522] MTRR: mtrr_disable: CPU0, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765474] MTRR: mtrr_disable: CPU1, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765207] MTRR: mtrr_disable: CPU54, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765225] MTRR: mtrr_disable: CPU8, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765282] MTRR: mtrr_disable: CPU88, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765150] MTRR: mtrr_disable: CPU119, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765370] MTRR: mtrr_disable: CPU49, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765395] MTRR: mtrr_disable: CPU16, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765348] MTRR: mtrr_disable: CPU52, MSR_MTRRdefType: 0x0, read: (0x0:0)
    > [ 0.765270] MTRR: mtrr_disable: CPU58, MSR_MTRRdefType: 0x0, read: (0x0:0)
    >
    > which basically hints at some speculative fun where we end up reading
    > the MSR *after* the write to it has already happened. After this thing:
    >
    > /* Disable MTRRs, and set the default type to uncached */
    > mtrr_wrmsr(MSR_MTRRdefType, state->mtrr_deftype_lo & ~0xcff,
    > state->mtrr_deftype_hi);
    >
    > and thus when we read it, we already read the disabled state. But this
    > is only a conjecture because I still have no clear idea how TF would
    > that even happen?!?

    Yeah, and why doesn't it happen when we handle only one cpu at a time?

    There might be some interaction between the cpus ...

    >
    > Needless to say, this fixes it, ofc:
    >
    > diff --git a/arch/x86/kernel/cpu/cacheinfo.c b/arch/x86/kernel/cpu/cacheinfo.c
    > index 3805a6d32d37..4a685898caf3 100644
    > --- a/arch/x86/kernel/cpu/cacheinfo.c
    > +++ b/arch/x86/kernel/cpu/cacheinfo.c
    > @@ -1116,12 +1116,14 @@ void cache_enable(struct cache_state *state)
    > __write_cr4(state->cr4);
    > }
    >
    > +static DEFINE_RAW_SPINLOCK(set_atomicity_lock);
    > +
    > static void cache_cpu_init(void)
    > {
    > unsigned long flags;
    > struct cache_state state = { };
    >
    > - local_irq_save(flags);
    > + raw_spin_lock_irqsave(&set_atomicity_lock, flags);
    > cache_disable(&state);
    >
    > if (memory_caching_control & CACHE_MTRR)
    > @@ -1131,7 +1133,7 @@ static void cache_cpu_init(void)
    > pat_cpu_init();
    >
    > cache_enable(&state);
    > - local_irq_restore(flags);
    > + raw_spin_unlock_irqrestore(&set_atomicity_lock, flags);
    > }
    >
    > static bool cache_aps_delayed_init = true;
    >
    > ---
    >
    > and frankly, considering how we have bigger fish to fry, I'd say we do
    > it the old way and leave that can'o'worms half-opened.

    I agree to keep this patch out of the series for now.

    >
    > Unless you wanna continue poking at it. I can give you access to that
    > box at work...

    Yes, please. I suspect there are some additional requirements for updating
    MTRR in parallel, or this is "just" a cpu bug.


    Juergen

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