Re: [PATCH 07/13] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_sept_add() to add SEPT pages

Yan Zhao
2025-01-08T02:16:15Z

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 :)