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Message-ID: <20230712165912.GA3100142@hirez.programming.kicks-ass.net>
Date: Wed, 12 Jul 2023 18:59:12 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kai Huang <kai.huang@...el.com>
Cc: kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
dave.hansen@...el.com, tglx@...utronix.de, bp@...en8.de,
mingo@...hat.com, hpa@...or.com, x86@...nel.org, seanjc@...gle.com,
pbonzini@...hat.com, kvm@...r.kernel.org, isaku.yamahata@...el.com,
sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
TDCALL/SEAMCALL leafs
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.
> > + xorq %rdi, %rdi
> > + .endif /* \ret && \host */
> > +
> > + /* Restore callee-saved GPRs as mandated by the x86_64 ABI */
> > + popq %r15
> > + popq %r14
> > + popq %r13
> > + popq %r12
> > + popq %rbx
> > + .endif /* \saved */
> >
> > FRAME_END
> > RET
> > --
> > 2.41.0
> >
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