On Wed, 2023-07-12 at 18:59 +0200, Peter Zijlstra wrote:
> On Wed, Jul 12, 2023 at 06:53:37PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 12, 2023 at 08:55:21PM +1200, Kai Huang wrote:
> >
> >
> > > @@ -72,7 +142,46 @@
> > > movq %r9, TDX_MODULE_r9(%rsi)
> > > movq %r10, TDX_MODULE_r10(%rsi)
> > > movq %r11, TDX_MODULE_r11(%rsi)
> > > - .endif
> > > + .endif /* \ret */
> > > +
> > > + .if \saved
> > > + .if \ret && \host
> > > + /*
> > > + * Clear registers shared by guest for VP.ENTER to prevent
> > > + * speculative use of guest's values, including those are
> > > + * restored from the stack.
> > > + *
> > > + * See arch/x86/kvm/vmx/vmenter.S:
> > > + *
> > > + * In theory, a L1 cache miss when restoring register from stack
> > > + * could lead to speculative execution with guest's values.
> > > + *
> > > + * Note: RBP/RSP are not used as shared register. RSI has been
> > > + * restored already.
> > > + *
> > > + * XOR is cheap, thus unconditionally do for all leafs.
> > > + */
> > > + xorq %rcx, %rcx
> > > + xorq %rdx, %rdx
> > > + xorq %r8, %r8
> > > + xorq %r9, %r9
> > > + xorq %r10, %r10
> > > + xorq %r11, %r11
> >
> > > + xorq %r12, %r12
> > > + xorq %r13, %r13
> > > + xorq %r14, %r14
> > > + xorq %r15, %r15
> > > + xorq %rbx, %rbx
> >
> > ^ those are an instant pop below, seems daft to clear them.
>
> Also, please use the 32bit variant:
>
> xorl %ecx, %ecx
>
> saves a RAX prefix each.
Sorry I am ignorant here. Won't "clearing ECX only" leave high bits of
registers still containing guest's value?
I see KVM code uses:
xor %eax, %eax
xor %ecx, %ecx
xor %edx, %edx
xor %ebp, %ebp
xor %esi, %esi
xor %edi, %edi
#ifdef CONFIG_X86_64
xor %r8d, %r8d
xor %r9d, %r9d
xor %r10d, %r10d
xor %r11d, %r11d
xor %r12d, %r12d
xor %r13d, %r13d
xor %r14d, %r14d
xor %r15d, %r15d
#endif
Which makes sense because KVM wants to support 32-bit too.
However for TDX is 64-bit only.
And I also see the current TDVMCALL code has:
xor %r8d, %r8d
xor %r9d, %r9d
xor %r10d, %r10d
xor %r11d, %r11d
xor %rdi, %rdi
xor %rdx, %rdx
Why does it need to use "d" postfix for all r* registers?
Sorry for those questions but I struggled when I wrote those assembly and am
hoping to get my mind cleared on this. :-)
Thanks!
