Re: [PATCH v3 4/5] x86/tdx: tdx_mcall_get_report0: Return -EBUSY on TDCALL_OPERAND_BUSY error

[Dan Williams]

Cedric Xing wrote:
> Return `-EBUSY` from tdx_mcall_get_report0() when `TDG.MR.REPORT` returns
> `TDCALL_OPERAND_BUSY`. This enables the caller to retry obtaining a
> TDREPORT later if another VCPU is extending an RTMR concurrently.

Can this not be prevented by proper locking? Otherwise this type of
collision sounds like a kernel bug, not something that should escape to
userspace.

I.e. userspace can not reasonably know when it is safe to retry, so take
locks to ensure forward progress.