Re: [PATCH v5 6/6] x86/tdx: Implement MOVS for MMIO

[Kirill A. Shutemov]

On Thu, Aug 29, 2024 at 08:40:56PM +0200, Alexey Gladkov wrote:
> On Thu, Aug 29, 2024 at 03:44:55PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Aug 28, 2024 at 12:44:36PM +0200, Alexey Gladkov wrote:
> > > From: "Alexey Gladkov (Intel)" <legion@xxxxxxxxxx>
> > > 
> > > Add emulation of the MOVS instruction on MMIO regions. MOVS emulation
> > > consists of dividing it into a series of read and write operations,
> > > which in turn will be validated separately.
> > > 
> > > This implementation is based on the same principle as in SEV. It splits
> > > MOVS into separate read and write operations, which in turn can cause
> > > nested #VEs depending on which of the arguments caused the first #VE.
> > > 
> > > The difference with the SEV implementation is the execution context. SEV
> > > code is executed in atomic context. Exception handler in TDX is executed
> > > with interrupts enabled. That's why the approach to locking is
> > > different. In TDX, mmap_lock is taken to verify and emulate the
> > > instruction.
> > > 
> > > Another difference is how the read and write instructions are executed
> > > for MOVS emulation. While in SEV each read/write operation returns to
> > > user space, in TDX these operations are performed from the kernel
> > > context.
> > 
> > It looks like SEV only returns to userspace to retry the instruction after
> > stepping on failed __get_user()/__put_user(), unrolling back to
> > vc_raw_handle_exception() and handling page fault there.
> 
> In vc_handle_mmio_movs() if regs->cx is not zero we return ES_RETRY. The
> vc_handle_mmio(), vc_handle_exitcode() return it as is. In
> vc_raw_handle_exception() if vc_handle_exitcode() returns ES_RETRY then we
> just return true. So, the ES_RETRY is not further visible.
> 
> Or am I missing something?

You are right. I didn't see this codepath.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov