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Wed, 11 Jul 2018 15:33:09 +0000 Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id w6BFX7P5018927 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 11 Jul 2018 15:33:08 GMT Received: from abhmp0013.oracle.com (abhmp0013.oracle.com [141.146.116.19]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id w6BFX7tm027663; Wed, 11 Jul 2018 15:33:07 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Wed, 11 Jul 2018 08:33:06 -0700 Subject: Re: [RFC PATCH] perf/core: don't sample kernel regs upon skid To: Mark Rutland Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , Jin Yao References: <20180702151250.14536-1-mark.rutland@arm.com> <20180702154655.GR2494@hirez.programming.kicks-ass.net> <20180702160249.qck45h76galxrckn@lakrids.cambridge.arm.com> <20180711055927.wcfcfkninfjwox3n@salmiak> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20180711055927.wcfcfkninfjwox3n@salmiak> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8950 signatures=668706 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=646 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110165 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/11/2018 01:59 AM, Mark Rutland wrote: > On Mon, Jul 09, 2018 at 06:42:29PM -0400, Boris Ostrovsky wrote: >> On 07/02/2018 12:02 PM, Mark Rutland wrote: >>> On Mon, Jul 02, 2018 at 05:46:55PM +0200, Peter Zijlstra wrote: >>>> On Mon, Jul 02, 2018 at 04:12:50PM +0100, Mark Rutland wrote: >>>>> +static struct pt_regs *perf_get_sample_regs(struct perf_event *event, >>>>> + struct pt_regs *regs) >>>>> +{ >>>>> + /* >>>>> + * Due to interrupt latency (AKA "skid"), we may enter the kernel >>>>> + * before taking an overflow, even if the PMU is only counting user >>>>> + * events. >>>>> + * >>>>> + * If we're not counting kernel events, always use the user regs when >>>>> + * sampling. >>>>> + * >>>>> + * TODO: what do we do about sampling a guest's registers? The IP is >>>>> + * special-cased, but for the rest of the regs they'll get the >>>>> + * user/kernel regs depending on whether exclude_kernel is set, which >>>>> + * is nonsensical. >>>>> + * >>>>> + * We can't get at the full set of regs in all cases (e.g. Xen's PV PMU >>>>> + * can't provide the GPRs), so should we just zero the GPRs when in a >>>>> + * guest? Or skip outputting the regs in perf_output_sample? >>>> Seems daft Xen cannot provide registers; why is that? Boris? >>> The xen_pmu_regs structure simply doesn't have them, so I assume there's >>> no API to get them. >>> >>> Given we don't currently sample the guest regs, I'd be tempted to just >>> zero them for now, or skip the sample at output time (if that doesn't >>> break some other case). >> (Was out on vacation, couldn't respond earlier) >> >> Yes, PV guests only get a limited set of registers passed to the handler >> by the hypervisor. GPRs are not part of this set. > Is that also true for Dom0? Yes. As far as VPMU is concerned there is (almost) no differences between privileged (i.e. dom0) and unprivileged PV guests. -boris