Messages in this thread | | | Date | Wed, 2 Feb 2022 15:48:30 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCHv2.1 05/29] x86/tdx: Add HLT support for TDX guests |
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On Tue, Feb 01, 2022 at 10:21:58PM +0100, Thomas Gleixner wrote: > On Sun, Jan 30 2022 at 01:30, Kirill A. Shutemov wrote: > > +/* > > + * Used in __tdx_hypercall() to determine whether to enable interrupts > > + * before issuing TDCALL for the EXIT_REASON_HLT case. > > + */ > > +#define ENABLE_IRQS_BEFORE_HLT 0x01 > > + > > /* > > * __tdx_module_call() - Used by TDX guests to request services from > > * the TDX module (does not include VMM services). > > @@ -230,6 +237,30 @@ SYM_FUNC_START(__tdx_hypercall) > > > > movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx > > > > + /* > > + * For the idle loop STI needs to be called directly before > > + * the TDCALL that enters idle (EXIT_REASON_HLT case). STI > > + * instruction enables interrupts only one instruction later. > > + * If there is a window between STI and the instruction that > > + * emulates the HALT state, there is a chance for interrupts to > > + * happen in this window, which can delay the HLT operation > > + * indefinitely. Since this is the not the desired result, > > + * conditionally call STI before TDCALL. > > + * > > + * Since STI instruction is only required for the idle case > > + * (a special case of EXIT_REASON_HLT), use the r15 register > > + * value to identify it. Since the R15 register is not used > > + * by the VMM as per EXIT_REASON_HLT ABI, re-use it in > > + * software to identify the STI case. > > + */ > > + cmpl $EXIT_REASON_HLT, %r11d > > + jne .Lskip_sti > > + cmpl $ENABLE_IRQS_BEFORE_HLT, %r15d > > + jne .Lskip_sti > > + /* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */ > > + xor %r15, %r15 > > + sti > > +.Lskip_sti: > > tdcall > > This really can be simplified: > > cmpl $EXIT_REASON_SAFE_HLT, %r11d > jne .Lnohalt > movl $EXIT_REASON_HLT, %r11d > sti > .Lnohalt: > tdcall > > and the below becomes: > > static bool tdx_halt(void) > { > return !!__tdx_hypercall(EXIT_REASON_HLT, !!irqs_disabled(), 0, 0, 0, NULL); > } > > void __cpuidle tdx_safe_halt(void) > { > if (__tdx_hypercall(EXIT_REASON_SAFE_HLT, 0, 0, 0, 0, NULL) > WARN_ONCE(1, "HLT instruction emulation failed\n"); > } > > Hmm?
EXIT_REASON_* are architectural, see SDM vol 3D, appendix C. There's no EXIT_REASON_SAFE_HLT.
Do you want to define a synthetic one? Like
#define EXIT_REASON_SAFE_HLT 0x10000
?
Looks dubious to me, I donno. I worry about possible conflicts with the spec in the future.
-- Kirill A. Shutemov
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