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Re: [Xen-devel] [PATCH 10/10] x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible
- To: Wei Liu <wei.liu2@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 16 May 2018 12:27:29 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- Delivery-date: Wed, 16 May 2018 11:27:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 14/05/18 16:48, Wei Liu wrote:
> On Fri, May 11, 2018 at 11:38:14AM +0100, Andrew Cooper wrote:
>> If Xen is virtualising MSR_SPEC_CTRL handling for guests, but using 0 as its
>> own MSR_SPEC_CTRL value, spec_ctrl_{enter,exit}_idle() need not write to the
>> MSR.
>>
>> Requested-by: Jan Beulich <JBeulich@xxxxxxxx>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Juergen Gross <jgross@xxxxxxxx>
>> ---
>> xen/arch/x86/spec_ctrl.c | 4 ++++
>> xen/include/asm-x86/cpufeatures.h | 1 +
>> xen/include/asm-x86/spec_ctrl.h | 8 ++------
>> 3 files changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
>> index a2328bd..f4a3165 100644
>> --- a/xen/arch/x86/spec_ctrl.c
>> +++ b/xen/arch/x86/spec_ctrl.c
>> @@ -526,6 +526,10 @@ void __init init_speculation_mitigations(void)
>> /* (Re)init BSP state now that default_spec_ctrl_flags has been
>> calculated. */
>> init_shadow_spec_ctrl_state();
>>
>> + /* If Xen is using any MSR_SPEC_CTRL settings, adjust the idle path. */
>> + if ( default_xen_spec_ctrl )
>> + setup_force_cpu_cap(X86_FEATURE_SC_MSR_IDLE);
>> +
>> xpti_init_default(false);
>> if ( opt_xpti == 0 )
>> setup_force_cpu_cap(X86_FEATURE_NO_XPTI);
>> diff --git a/xen/include/asm-x86/cpufeatures.h
>> b/xen/include/asm-x86/cpufeatures.h
>> index 9d5d81e..b90aa2d 100644
>> --- a/xen/include/asm-x86/cpufeatures.h
>> +++ b/xen/include/asm-x86/cpufeatures.h
>> @@ -31,3 +31,4 @@ XEN_CPUFEATURE(SC_MSR_HVM, (FSCAPINTS+0)*32+17) /*
>> MSR_SPEC_CTRL used by Xe
>> XEN_CPUFEATURE(SC_RSB_PV, (FSCAPINTS+0)*32+18) /* RSB overwrite
>> needed for PV */
>> XEN_CPUFEATURE(SC_RSB_HVM, (FSCAPINTS+0)*32+19) /* RSB overwrite
>> needed for HVM */
>> XEN_CPUFEATURE(NO_XPTI, (FSCAPINTS+0)*32+20) /* XPTI mitigation not
>> in use */
>> +XEN_CPUFEATURE(SC_MSR_IDLE, (FSCAPINTS+0)*32+21) /* (SC_MSR_PV ||
>> SC_MSR_HVM) && default_xen_spec_ctrl */
> I don't follow: the code above only depends on default_xen_spec_ctrl but
> the comment says SC_MSR_PV and SC_MSR_HVM are also taken into
> consideration.
default_xen_spec_ctrl is only ever nonzero when SC_MSR_PV or SC_MSR_HVM
is set. In principle, I could assert that one of the two is set.
I have phrased it like this because it is going to become rather more
complicated (than != 0) when adding IBRS_ATT mode.
~Andrew
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