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Date:   Tue, 16 Jun 2020 12:33:13 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Christoph Hellwig <hch@lst.de>, Dexuan Cui <decui@microsoft.com>,
        vkuznets <vkuznets@redhat.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andy Lutomirski <luto@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Michael Kelley <mikelley@microsoft.com>,
        Ju-Hyoung Lee <juhlee@microsoft.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: hv_hypercall_pg page permissios
Message-ID: <20200616103313.GA30833@lst.de>
References: <C311EB52-A796-4B94-AADD-CCABD19B377E@amacapital.net> <HK0P153MB0322D52F61E540CA7515CC4BBF810@HK0P153MB0322.APCP153.PROD.OUTLOOK.COM> <87y2ooiv5k.fsf@vitty.brq.redhat.com> <HK0P153MB0322DE798AA39BCCD4A208E4BF9C0@HK0P153MB0322.APCP153.PROD.OUTLOOK.COM> <HK0P153MB0322EB3EE51073CC021D4AEABF9C0@HK0P153MB0322.APCP153.PROD.OUTLOOK.COM> <20200616072318.GA17600@lst.de> <20200616101807.GO2531@hirez.programming.kicks-ass.net> <20200616102350.GA29684@lst.de> <20200616102412.GB29684@lst.de> <20200616103137.GQ2531@hirez.programming.kicks-ass.net>
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On Tue, Jun 16, 2020 at 12:31:37PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 16, 2020 at 12:24:12PM +0200, Christoph Hellwig wrote:
> > On Tue, Jun 16, 2020 at 12:23:50PM +0200, Christoph Hellwig wrote:
> 
> > > > +	hv_hypercall_pg = module_alloc(PAGE_SIZE);
> > > >  	if (hv_hypercall_pg == NULL) {
> > > >  		wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
> > > >  		goto remove_cpuhp_state;
> > > >  	}
> > > >  
> > > > +	set_memory_ro((unsigned long)hv_hypercall_pg, 1);
> > > > +	set_memory_x((unsigned long)hv_hypercall_pg, 1);
> > > 
> > > The changing of the permissions sucks.  I thought about adding
> > > a module_alloc_prot with an explicit pgprot_t argument.  On x86
> > > alone at least ftrace would also benefit from that.
> 
> How would ftrace benefit from a RX permission? We need to actually write
> to the page first. This HV thing is special in that it lets the host
> write.

sorry, s/ftrace/kprobes/.  See my updated branch here:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/module_alloc-cleanup