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

[Sathyanarayanan Kuppuswamy]

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