linux-kernel - Re: [PATCH 07/13] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_sept

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Message-ID: <Z33fmRV5x8QNQtXZ@yzhao56-desk.sh.intel.com>
Date: Wed, 8 Jan 2025 10:14:49 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "Hansen, Dave" <dave.hansen@...el.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "sean.j.christopherson@...el.com"
	<sean.j.christopherson@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 07/13] x86/virt/tdx: Add SEAMCALL wrapper
 tdh_mem_sept_add() to add SEPT pages

On Wed, Jan 08, 2025 at 09:43:37AM +0800, Edgecombe, Rick P wrote:
> On Tue, 2025-01-07 at 17:20 -0800, Dave Hansen wrote:
> > On 1/7/25 17:12, Yan Zhao wrote:
> > > So, why does this bitfields definition make things worse?
> > 
> > Look at the kernel page table management. Why don't we use bitfields for
> > _that_? Look at the link I sent. Bitfields can cause some really goofy
> > unexpected behavior if you pass them around like they were a full type.
> 
> Huh, so this enum is unsafe for reading out the individual fields because if
> shifting them, it will perform the shift with the size of the source bit field
> size. It is safe in the way it is being used in these patches, which is to
> encode a u64. But if we ever started to use tdx_sept_gpa_mapping_info to process
> output from a SEAMCALL, or something, we could set ourselves up for the same
> problem as the SEV bug.
Thanks for the explanation!
Sorry that I didn't clearly explain the usage to Dave.

> Let's not open code the encoding in each SEAMCALL though. What about replacing
> it with just a helper that encodes the u64 gpa from two args: gfn and tdx_level.
> We could add some specific over-size behavior for the fields, but I'd think it
> would be ok to keep it simple. Maybe something like this:
> 
> static u64 encode_gpa_mapping_info(gfn_t gfn, unsigned int tdx_level)
> {
> 	u64 val = 0;
> 
> 	val |= level;
> 	val |= gfn << TDX_MAPPING_INFO_GFN_SHIFT;
> 
> 	return val;
> }
That's a clever alternative :)