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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=LUn/Q21ek7qt428VJCVaK7r0J65rS474/tbeyN6jnxDuA4e+VPH4XEfI/OsSLUYBB BvpRAzADaXbAdlHcAxsKfc87VmEQu4veJ4shAXxl6d+KfdMZGS/rDLljsWBue+tMlb jvEyu1Yv4F+eoj5cW4qEigccihTAn74hoJfZFM3aAPaGNzM7K5G9tj8QnwXy8c/25/ yT845Q0l6v0MeAUuXA9Fh8iSxmX1i65xs7b1QnacLeCOR9am33RZQnJmKye1tVbACZ 56XAfIsHBaouAtfIE0fwp1tmTpu1+KQweyH6OVB1NQwNJmnyPtnyxqbNHrqwDH/2lO 6Ooz8AMRP9cKQ== Message-ID: Date: Thu, 24 Apr 2025 01:06:56 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 21/34] x86/msr: Utilize the alternatives mechanism to write MSR To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, linux-pm@vger.kernel.org, linux-edac@vger.kernel.org, xen-devel@lists.xenproject.org, linux-acpi@vger.kernel.org, linux-hwmon@vger.kernel.org, netdev@vger.kernel.org, platform-driver-x86@vger.kernel.org Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, acme@kernel.org, andrew.cooper3@citrix.com, peterz@infradead.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, wei.liu@kernel.org, ajay.kaher@broadcom.com, bcm-kernel-feedback-list@broadcom.com, tony.luck@intel.com, pbonzini@redhat.com, vkuznets@redhat.com, seanjc@google.com, luto@kernel.org, boris.ostrovsky@oracle.com, kys@microsoft.com, haiyangz@microsoft.com, decui@microsoft.com References: <20250422082216.1954310-1-xin@zytor.com> <20250422082216.1954310-22-xin@zytor.com> <37c88ea3-dd24-4607-9ee1-0f19025aaef3@suse.com> Content-Language: en-US From: Xin Li Autocrypt: addr=xin@zytor.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 4/23/2025 9:05 AM, Jürgen Groß wrote: >> It's not a major change, but when it is patched to use the immediate >> form MSR write instruction, it's straightforwardly streamlined. > > It should be rather easy to switch the current wrmsr/rdmsr paravirt > patching > locations to use the rdmsr/wrmsr instructions instead of doing a call to > native_*msr(). > > The case of the new immediate form could be handled the same way. Actually, that is how we get this patch with the existing alternatives infrastructure. And we took a step further to also remove the pv_ops MSR APIs... It looks to me that you want to add a new facility to the alternatives infrastructure first? >>> Only the "paravirt" term has been eliminated. >> >> Yes. >> >> But a PV guest doesn't operate at the highest privilege level, which >> means MSR instructions typically result in a #GP fault.  I actually >> think the pv_ops MSR APIs are unnecessary because of this inherent >> limitation. >> >> Looking at the Xen MSR code, except PMU and just a few MSRs, it falls >> back to executes native MSR instructions.  As MSR instructions trigger >> #GP, Xen takes control and handles them in 2 ways: >> >>    1) emulate (or ignore) a MSR operation and skip the guest instruction. >> >>    2) inject the #GP back to guest OS and let its #GP handler handle it. >>       But Linux MSR exception handler just ignores the MSR instruction >>       (MCE MSR exception will panic). >> >> So why not let Xen handle all the details which it already tries to do? > > Some MSRs are not handled that way, but via a kernel internal emulation. > And those are handled that way mostly due to performance reasons. And some > need special treatment. > >> (Linux w/ such a change may not be able to run on old Xen hypervisors.) > > Yes, and this is something to avoid. > > And remember that Linux isn't the only PV-mode guest existing. > >> BTW, if performance is a concern, writes to MSR_KERNEL_GS_BASE and >> MSR_GS_BASE anyway are hpyercalls into Xen. > > Yes, and some other MSR writes are just NOPs with Xen-PV. > I will do some cleanup and refactor first. BTW, at least we can merge the safe() APIs into the non-safe() ones. Thanks! Xin