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Subject: Re: [PATCH v3 09/21] x86/virt/tdx: Get information about TDX module
 and convertible memory
From:   Kai Huang <kai.huang@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     seanjc@google.com, pbonzini@redhat.com, len.brown@intel.com,
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        reinette.chatre@intel.com, dan.j.williams@intel.com,
        peterz@infradead.org, ak@linux.intel.com,
        kirill.shutemov@linux.intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com,
        isaku.yamahata@intel.com
Date:   Wed, 25 May 2022 16:57:35 +1200
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On Wed, 2022-05-25 at 16:47 +1200, Kai Huang wrote:
> On Fri, 2022-04-29 at 10:47 -0700, Dave Hansen wrote:
> > On 4/28/22 16:14, Kai Huang wrote:
> > > On Thu, 2022-04-28 at 07:06 -0700, Dave Hansen wrote:
> > > > On 4/27/22 17:15, Kai Huang wrote:
> > > > > > Couldn't we get rid of that comment if you did something like:
> > > > > > 
> > > > > > 	ret = tdx_get_sysinfo(&tdx_cmr_array, &tdx_sysinfo);
> > > > > 
> > > > > Yes will do.
> > > > > 
> > > > > > and preferably make the variables function-local.
> > > > > 
> > > > > 'tdx_sysinfo' will be used by KVM too.
> > > > 
> > > > In other words, it's not a part of this series so I can't review whether
> > > > this statement is correct or whether there's a better way to hand this
> > > > information over to KVM.
> > > > 
> > > > This (minor) nugget influencing the design also isn't even commented or
> > > > addressed in the changelog.
> > > 
> > > TDSYSINFO_STRUCT is 1024B and CMR array is 512B, so I don't think it should be
> > > in the stack.  I can change to use dynamic allocation at the beginning and free
> > > it at the end of the function.  KVM support patches can change it to static
> > > variable in the file.
> > 
> > 2k of stack is big, but it isn't a deal breaker for something that's not
> > nested anywhere and that's only called once in a pretty controlled
> > setting and not in interrupt context.  I wouldn't cry about it.
> 
> Hi Dave,
> 
> I got below complaining when I use local variable for TDSYSINFO_STRUCT and CMR
> array:
> 
> arch/x86/virt/vmx/tdx/tdx.c:383:1: warning: the frame size of 3072 bytes is
> larger than 1024 bytes [-Wframe-larger-than=]
>   383 | }
> 
> So I don't think we can use local variable for them.  I'll still use static
> variables to avoid dynamic allocation.  In the commit message, I'll explain they
> are too big to put into the stack, and KVM will need to use TDSYSINFO_STRUCT
> reported by TDX module anyway.
> 
> Let me know if you don't agree?

Btw, CMR array alone can be put into the stack.  It will never be used by KVM,
so I'll put CMR array as local variable, but keep tdx_sysinfo as static
variable.

-- 
Thanks,
-Kai