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Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call
- To: Borislav Petkov <bp@xxxxxxxxx>
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Wed, 2 Dec 2020 15:48:21 +0100
- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, peterz@xxxxxxxxxxxxx, luto@xxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Deep Shah <sdeep@xxxxxxxxxx>, "VMware, Inc." <pv-drivers@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Wed, 02 Dec 2020 14:48:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.12.20 13:32, Borislav Petkov wrote:
On Fri, Nov 20, 2020 at 12:46:22PM +0100, Juergen Gross wrote:
@@ -123,12 +115,15 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe,
SYM_L_GLOBAL)
* Try to use SYSRET instead of IRET if we're returning to
* a completely clean 64-bit userspace context. If we're not,
* go to the slow exit path.
+ * In the Xen PV case we must use iret anyway.
*/
- movq RCX(%rsp), %rcx
- movq RIP(%rsp), %r11
- cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
- jne swapgs_restore_regs_and_return_to_usermode
+ ALTERNATIVE __stringify( \
+ movq RCX(%rsp), %rcx; \
+ movq RIP(%rsp), %r11; \
+ cmpq %rcx, %r11; /* SYSRET requires RCX == RIP */ \
+ jne swapgs_restore_regs_and_return_to_usermode), \
+ "jmp swapgs_restore_regs_and_return_to_usermode",
X86_FEATURE_XENPV
Why such a big ALTERNATIVE when you can simply do:
/*
* Try to use SYSRET instead of IRET if we're returning to
* a completely clean 64-bit userspace context. If we're not,
* go to the slow exit path.
* In the Xen PV case we must use iret anyway.
*/
ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode",
X86_FEATURE_XENPV
movq RCX(%rsp), %rcx;
movq RIP(%rsp), %r11;
cmpq %rcx, %r11; /* SYSRET requires RCX == RIP */ \
jne swapgs_restore_regs_and_return_to_usermode
?
I wanted to avoid the additional NOPs for the bare metal case.
If you don't mind them I can do as you are suggesting.
Juergen
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