linux-kernel - Re: [PATCH v3 09/21] x86/virt/tdx: Get information about TDX module and convertible memory

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Date:   Thu, 26 May 2022 04:00:49 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v3 09/21] x86/virt/tdx: Get information about TDX module
 and convertible memory

On Wed, 2022-05-25 at 16:57 +1200, Kai Huang wrote:
> 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.
> 

Sorry for multiple emails about this.  If I put CMR array to the stack, I still
sometimes get the build warning.  So will use static variables.

Also, constructing TDMRs internally needs to use tdx_sysinfo (max_tdmrs,
pamt_entry_size, max_rsvd_per_tdmr), so with static variable they don't need to
be passed around as function arguments.

-- 
Thanks,
-Kai