linux-kernel - RE: [PATCH v4] hyperv: root partition faults writing to VP ASSIST MSR PAGE

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Date:   Sat, 31 Jul 2021 00:12:45 +0000
From:   Sunil Muthuswamy <sunilmut@...rosoft.com>
To:     Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "viremana@...ux.microsoft.com" <viremana@...ux.microsoft.com>,
        "nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>
Subject: RE: [PATCH v4] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE

>  	hv_common_cpu_die(cpu);
> 
> -	if (hv_vp_assist_page && hv_vp_assist_page[cpu])
> -		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, 0);
> +	if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
> +		union hv_vp_assist_msr_contents msr = {0};
> +		if (hv_root_partition) {
> +			/*
> +			 * For Root partition the VP ASSIST page is mapped to
> +			 * hypervisor provided page, and thus, we unmap the
> +			 * page here and nullify it, so that in future we have
> +			 * correct page address mapped in hv_cpu_init.
> +			 */
> +			memunmap(hv_vp_assist_page[cpu]);
> +			hv_vp_assist_page[cpu] = NULL;
> +			rdmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);

If the vp assist page was previously enabled for root, it will continue to stay enabled
when you 'wrmsr' below. We need to explicitly set the enable bit to 0.

> +		}
> +		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> +	}