Re: [Xen-devel] [PATCH 2/2] x86/hvm: Corrections to RDTSCP intercept handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.11.18 at 18:07, <andrew.cooper3@xxxxxxxxxx> wrote:
> For both VT-x and SVM, the RDTSCP intercept will trigger if the pipeline
> supports the instruction, but the guest may have not have rdtscp in its
> featureset.  Bring the vmexit handlers in line with the main emulator
> behaviour by optionally handing back #UD.
> 
> Next on the AMD side, if RDTSCP actually ends up being intercepted on a debug
> build, we first update regs->rcx, then call __get_instruction_length() asking
> for RDTSC.  As the two instructions are different (and indeed, different
> lengths!), __get_instruction_length_from_list() fails and hands back a #GP
> fault.
> 
> This can demonstrated by putting a guest into tsc_mode="always emulate" and
> executing an rdtscp instruction:
> 
>   (d1) --- Xen Test Framework ---
>   (d1) Environment: HVM 64bit (Long mode 4 levels)
>   (d1) Test rdtscp
>   (d1) TSC mode 1
>   (XEN) emulate.c:159:d1v0 __get_instruction_length_from_list: Mismatch 
> between expected and actual instruction:
>   (XEN) emulate.c:163:d1v0   list[0] val 8, { opc 0xf0031, modrm 0 }, list 
> entries: 1
>   (XEN) emulate.c:165:d1v0   rip 0x10475f, nextrip 0x104762, len 3
>   (XEN) Insn_len emulation failed (1): d1v0 64bit @ 0008:0010475f -> 0f 01 f9 
> 5b 31 ff 31 c0 e9 c4 db ff ff 00 00 00
>   (d1) ******************************
>   (d1) PANIC: Unhandled exception at 0008:000000000010475f
>   (d1) Vec 13 #GP[0000]
>   (d1) ******************************
> 
> First, teach __get_instruction_length() to cope with RDTSCP, and improve
> svm_vmexit_do_rdtsc() to ask for the correct instruction.  Move the regs->rcx
> adjustment into this function to ensure it gets done after we are done
> potentially raising faults.
> 
> Reported-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with or without Paul's additional remark addressed.

Jan



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