linux-kernel - Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

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Date:   Mon, 11 Apr 2022 16:38:33 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Mark Gross <mgross@...ux.intel.com>
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
        platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest
 attestation interface driver

Hi,

On 4/4/22 21:56, Sathyanarayanan Kuppuswamy wrote:
> Hi Hans,
> 
> On 4/4/22 3:07 AM, Hans de Goede wrote:
>>> +static int __init tdx_attest_init(void)
>>> +{
>>> +    dma_addr_t handle;
>>> +    long ret = 0;
>>> +
>>> +    mutex_lock(&attestation_lock);
>>> +
>>> +    ret = misc_register(&tdx_attest_device);
>>> +    if (ret) {
>>> +        pr_err("misc device registration failed\n");
>>> +        mutex_unlock(&attestation_lock);
>>> +        return ret;
>>> +    }
>> Why not do this as the last thing of the probe?
> 
> We need misc device reference in dma_alloc_coherent() and
> dma_set_coherent_mask() calls. This is the reason for keeping
> misc_register() at the beginining of the init function.

Erm, you are supposed to pass an actual device-device as
parameter to dma_alloc_coherent(), so that it can see
what bus/dma-domain the device is connected to and if
an ioMMU might be involved...

>> That will avoid the need to unregister this again in all
>> the error-exit paths and also fixes a possible deadlock.
>>
> 
> Agree. But, unless we create another device locally, I don't
> think we can avoid this. Do you prefer this approach?

Yes, passing the "struct device" which is embedded inside
a miscdevice as device to dma_alloc_coherent() is just
wrong. Please make your module_init function register
a platform_device using platform_device_register_simple()
(on systems with TDX support) and then turn your code/driver
into a standard platform_driver using the platform_device
which the driver binds to as parameter to dma_alloc_coherent().

See: https://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git/commit/?id=c4d2ff35d350428eca2ae1a1e2119e4c6297811d

for a simple (completely unrelated) driver which uses this
method to have a device to bind to for talking to a secondary
function of the embedded-controller on some platforms.

Regards,

Hans