Hello, I am studing Xen hypercall now. I found that hypercall is invoked via hypercall_page, which is only filled with (in no-hypervisor-kernel-mode ): mov $i, %eax int $0x82 ret Why not invoked the hypercall directly by "int $0x82" ? What''s the advantage of using hypercall_page? Thanks, Wu _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
>From: Wu Bingzheng >Sent: Thursday, October 23, 2008 9:18 AM > >Hello, > >I am studing Xen hypercall now. I found that hypercall is invoked via >hypercall_page, which is only filled with (in >no-hypervisor-kernel-mode ): > >mov $i, %eax >int $0x82 >ret > >Why not invoked the hypercall directly by "int $0x82" ? What''s the >advantage of using hypercall_page? >This allows guest migrated to a newer/older xen with a different hypercall invocation convention. Xen fills hypercall page by its convention, and thus release guest from hardcoding specific flow. Thanks Kevin _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
As you know, the old Intel/AMD x86 cpus use INT to invoke kernel''s service. But the newer CPUs introduce two instruction pairs: syscall/sysret, syscenter/sysexit. So, because the hypercall page is filled by Xen, it can hide the difference of this two types. Guest OS only take one uniform format to invoke a hypercall. I gusee this is the reason. -Techie -----邮件原件----- 发件人: xen-devel-bounces@lists.xensource.com [mailto:xen-devel-bounces@lists.xensource.com] 代表 Wu Bingzheng 发送时间: 2008年10月23日 9:18 收件人: xen-devel@lists.xensource.com 主题: [Xen-devel] Why using hypercall_page ? Hello, I am studing Xen hypercall now. I found that hypercall is invoked via hypercall_page, which is only filled with (in no-hypervisor-kernel-mode ): mov $i, %eax int $0x82 ret Why not invoked the hypercall directly by "int $0x82" ? What''s the advantage of using hypercall_page? Thanks, Wu _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
To invoke hypercall:
1. Not using hypercall_page:
mov $__HYPERCALL_sched_op, %eax
int $0x82
2. Using hypercall_page:
call hypercall_page + __HYPERCALL_sched_op * 32
Besides, using the hypercall_page, guest has to keep a page for hypercall_page.
I do not think the hypercall_page makes the thing much easier.
Is "release guest from hardcoding" the only advantage of using
hypercall_page?
Thanks,
Wu
在 2008-10-23四的 09:24 +0800，Tian, Kevin写道：> >From: Wu Bingzheng
> >Sent: Thursday, October 23, 2008 9:18 AM
> >
> >Hello,
> >
> >I am studing Xen hypercall now. I found that hypercall is invoked via
> >hypercall_page, which is only filled with (in
> >no-hypervisor-kernel-mode ):
> >
> >mov $i, %eax
> >int $0x82
> >ret
> >
> >Why not invoked the hypercall directly by "int $0x82" ?
What''s the
> >advantage of using hypercall_page?
> >
>
> This allows guest migrated to a newer/older xen with a different
> hypercall invocation convention. Xen fills hypercall page by its
> convention, and thus release guest from hardcoding specific flow.
>
> Thanks
> Kevin
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I think that''s the reason too. Thank you very much. Wu 在 2008-10-23四的 09:31 +0800，Li Yaqiong写道：> As you know, the old Intel/AMD x86 cpus use INT to invoke kernel''s > service. But the newer CPUs introduce two instruction pairs: > syscall/sysret, syscenter/sysexit. So, because the hypercall page is > filled by Xen, it can hide the difference of this two types. Guest OS > only take one uniform format to invoke a hypercall. > I gusee this is the reason. > > -Techie > > -----邮件原件----- > 发件人: xen-devel-bounces@lists.xensource.com > [mailto:xen-devel-bounces@lists.xensource.com] 代表 Wu Bingzheng > 发送时间: 2008年10月23日 9:18 > 收件人: xen-devel@lists.xensource.com > 主题: [Xen-devel] Why using hypercall_page ? > > Hello, > > I am studing Xen hypercall now. I found that hypercall is invoked via > hypercall_page, which is only filled with (in > no-hypervisor-kernel-mode ): > > mov $i, %eax > int $0x82 > ret > > Why not invoked the hypercall directly by "int $0x82" ? What''s the > advantage of using hypercall_page? > > Thanks, > Wu > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel > >_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Yes, I think Kevin and Techie have explained it clearly. BTW, for HVM guest's hypercall, we don't use int 0x82 or the sysXXX instructions; we use VMCALL inside VMX guest or something similar (VMMCALL? I'm not sure) inside SVM guest. Even for PV guest, the hypercall stub codes may have different formats/versions... We can see these differences in the function hypercall_page_initialise(). So considering compatibility and portability, it's really not OK for a guest to assume the underlying stub codes or doing hard coding. Using the hypercall-page method, various guests can use one unified method to invoke hypercalls. Thanks, -- Dexuan -----Original Message----- From: xen-devel-bounces@lists.xensource.com [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of Wu Bingzheng Sent: 2008年10月23日 9:46 To: Li Yaqiong Cc: xen-devel@lists.xensource.com Subject: Re: 答复: [Xen-devel] Why using hypercall_page ? I think that's the reason too. Thank you very much. Wu 在 2008-10-23四的 09:31 +0800，Li Yaqiong写道：> As you know, the old Intel/AMD x86 cpus use INT to invoke kernel's > service. But the newer CPUs introduce two instruction pairs: > syscall/sysret, syscenter/sysexit. So, because the hypercall page is > filled by Xen, it can hide the difference of this two types. Guest OS > only take one uniform format to invoke a hypercall. > I gusee this is the reason. > > -Techie > > -----邮件原件----- > 发件人: xen-devel-bounces@lists.xensource.com > [mailto:xen-devel-bounces@lists.xensource.com] 代表 Wu Bingzheng > 发送时间: 2008年10月23日 9:18 > 收件人: xen-devel@lists.xensource.com > 主题: [Xen-devel] Why using hypercall_page ? > > Hello, > > I am studing Xen hypercall now. I found that hypercall is invoked via > hypercall_page, which is only filled with (in > no-hypervisor-kernel-mode ): > > mov $i, %eax > int $0x82 > ret > > Why not invoked the hypercall directly by "int $0x82" ? What's the > advantage of using hypercall_page? > > Thanks, > Wu > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel > >_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel