From: Thomas Gleixner <tglx@linutronix.de>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
chris@arachsys.com, Ingo Molnar <mingo@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH] x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only
Date: Wed, 24 Sep 2014 21:39:41 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.10.1409242136170.4604@nanos> (raw)
In-Reply-To: <1411384668-11135-1-git-send-email-pbonzini@redhat.com>
On Mon, 22 Sep 2014, Paolo Bonzini wrote:
> On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA.
> In that case, KVM will fail to patch VMCALL instructions to VMMCALL
> as required on AMD processors.
>
> The failure mode is currently a divide-by-zero exception, which obviously
> is a KVM bug that has to be fixed. However, picking the right instruction
> between VMCALL and VMMCALL will be faster and will help if you cannot upgrade
> the hypervisor.
>
> -/* This instruction is vmcall. On non-VT architectures, it will generate a
> - * trap that we will then rewrite to the appropriate instruction.
> +#ifdef CONFIG_DEBUG_RODATA
> +#define KVM_HYPERCALL \
> + ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL)
If we can do it via a feature bit and alternatives, then why do you
want to patch it manually if CONFIG_DEBUG_RODATA=n?
Just because more #ifdeffery makes the code more readable?
Thanks,
tglx
prev parent reply other threads:[~2014-09-24 19:39 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-22 11:17 [PATCH] x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only Paolo Bonzini
2014-09-22 19:43 ` Borislav Petkov
2014-09-23 8:00 ` Paolo Bonzini
2014-09-23 8:27 ` Borislav Petkov
2014-09-24 19:39 ` Thomas Gleixner [this message]
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