linux-kernel - Re: [PATCH 3/7] iommu/vt-d: Support Enhanced Command Interface

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Message-ID: <9b4f4602-55c6-40cd-5bf9-a47c16cec1a3@linux.intel.com>
Date:   Fri, 13 Jan 2023 14:02:35 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>, joro@...tes.org,
        will@...nel.org, dwmw2@...radead.org, robin.murphy@....com,
        robert.moore@...el.com, rafael.j.wysocki@...el.com,
        lenb@...nel.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] iommu/vt-d: Support Enhanced Command Interface



On 2023-01-13 9:12 a.m., Baolu Lu wrote:
> On 2023/1/13 21:55, Baolu Lu wrote:
>>> +/*
>>> + * Function to submit a command to the enhanced command interface. The
>>> + * valid enhanced command descriptions are defined in Table 47 of the
>>> + * VT-d spec. The VT-d hardware implementation may support some but not
>>> + * all commands, which can be determined by checking the Enhanced
>>> + * Command Capability Register.
>>> + *
>>> + * Return values:
>>> + *  - 0: Command successful without any error;
>>> + *  - Negative: software error value;
>>> + *  - Nonzero positive: failure status code defined in Table 48.
>>> + */
>>> +int ecmd_submit_sync(struct intel_iommu *iommu, u8 ecmd,
>>> +             u64 oa, bool has_ob, u64 ob)
>>> +{
>>> +    unsigned long flags;
>>> +    u64 res;
>>> +    int ret;
>>> +
>>> +    if (!cap_ecmds(iommu->cap))
>>> +        return -ENODEV;
>>> +
>>> +    raw_spin_lock_irqsave(&iommu->register_lock, flags);
>>> +
>>> +    res = dmar_readq(iommu->reg + DMAR_ECRSP_REG);
>>> +    if (res & DMA_ECMD_ECRSP_IP) {
>>> +        ret = -EBUSY;
>>> +        goto err;
>>> +    }
>>> +
>>> +    if (has_ob)
>>> +        dmar_writeq(iommu->reg + DMAR_ECEO_REG, ob);
>>
>> The ecmds that require a Operand B are statically defined in the spec,
>> right? What will it look like if we define a static ignore_ob(ecmd)?
> 

If so, I think we have to maintain a table of ecmd in the ignore_ob(),
and check the given ecmd at runtime, right?
That sounds hard to maintain and low efficiency with more and more ecmds
are introduced.

> Or simply remove has_ob parameter? The least case is an unnecessary
> write to a register. It's fine as far as I can see since we should avoid
> using it in any critical path.

I was told in the internal review that a MMIO write may trigger a VM
exit, if in a guest. We should avoid such unnecessary MMIO write.

For PMU, right, I don't think we use it at critical path. Now the PMU is
the only customer for ecmd. I think the extra MMIO write can be tolerant.

I will remove has_ob and add some comments in V2.

Thanks,
Kan

> 
> -- 
> Best regards,
> baolu