linux-kernel - Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface driver

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Message-ID: <4a24261c-a745-bd47-9d90-eb444f3b6399@linux.intel.com>
Date:   Thu, 28 Apr 2022 11:18:58 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Wander Lairson Costa <wander@...hat.com>
Cc:     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,
        "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>,
        Kai Huang <kai.huang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface
 driver



On 4/28/22 11:04 AM, Dave Hansen wrote:
> On 4/28/22 10:56, Sathyanarayanan Kuppuswamy wrote:
>> On 4/28/22 10:45 AM, Wander Lairson Costa wrote:
>>> On Fri, Apr 22, 2022 at 04:34:16PM -0700, Kuppuswamy Sathyanarayanan
>>> wrote:
>>>> +static long tdx_get_tdreport(void __user *argp)
>>>> +{
>>>> +    void *report_buf = NULL, *tdreport_buf = NULL;
>>>> +    long ret = 0, err;
>>>> +
>>>> +    /* Allocate space for report data */
>>>> +    report_buf = kmalloc(TDX_REPORT_DATA_LEN, GFP_KERNEL);
>>>> +    if (!report_buf)
>>>> +        return -ENOMEM;
>>>> +
>>>> +    /*
>>>> +     * Allocate space for TDREPORT buffer (1024-byte aligned).
>>>> +     * Full page alignment is more than enough.
>>>> +     */
>>>> +    tdreport_buf = (void *)get_zeroed_page(GFP_KERNEL);
>>>
>>> Maybe we should add BUILD_BUG_ON(TDX_TDREPORT_LEN > PAGE_SIZE)
>>
>> Currently, it is a constant value < PAGE_SIZE. But I can add the
>> BUILD_BUG_ON check for it.
> 
> That's kinda silly.  If it might ever be bigger than a page, you just do:
> 
> 	tdreport_buf = alloc_pages();
> 
> But, seriously, TDX_TDREPORT_LEN is part of the ABI and can't change.
> kmalloc() would work too since TDX_TDREPORT_LEN is a power of 2.


Agree. For our current requirement, kmalloc will just work fine. If the
length changes in the future, I can start using alloc_pages.

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer