linux-kernel - Re: [PATCH 4/6] x86,hyperv: Clean up hv_do

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <01c65464-8535-28d8-a9b5-eb4f90114e2d@gmail.com>
Date: Mon, 14 Apr 2025 16:06:41 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, jpoimboe@...nel.org,
 pawan.kumar.gupta@...ux.intel.com, seanjc@...gle.com, pbonzini@...hat.com,
 ardb@...nel.org, kees@...nel.org, Arnd Bergmann <arnd@...db.de>,
 gregkh@...uxfoundation.org, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-efi@...r.kernel.org, samitolvanen@...gle.com, ojeda@...nel.org
Subject: Re: [PATCH 4/6] x86,hyperv: Clean up hv_do_hypercall()



On 14. 04. 25 13:11, Peter Zijlstra wrote:
> What used to be a simple few instructions has turned into a giant mess
> (for x86_64). Not only does it use static_branch wrong, it mixes it
> with dynamic branches for no apparent reason.
> 
> Notably it uses static_branch through an out-of-line function call,
> which completely defeats the purpose, since instead of a simple
> JMP/NOP site, you get a CALL+RET+TEST+Jcc sequence in return, which is
> absolutely idiotic.
> 
> Add to that a dynamic test of hyperv_paravisor_present, something
> which is set once and never changed.
> 
> Replace all this idiocy with a single direct function call to the
> right hypercall variant.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>   arch/x86/hyperv/hv_init.c       |   21 ++++++
>   arch/x86/hyperv/ivm.c           |   14 ++++
>   arch/x86/include/asm/mshyperv.h |  137 +++++++++++-----------------------------
>   arch/x86/kernel/cpu/mshyperv.c  |   18 +++--
>   4 files changed, 88 insertions(+), 102 deletions(-)
> 
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -35,7 +35,28 @@
>   #include <linux/highmem.h>
>   
>   void *hv_hypercall_pg;
> +
> +#ifdef CONFIG_X86_64
> +u64 hv_pg_hypercall(u64 control, u64 param1, u64 param2)
> +{
> +	u64 hv_status;
> +
> +	if (!hv_hypercall_pg)
> +		return U64_MAX;
> +
> +	register u64 __r8 asm("r8") = param2;
> +	asm volatile (CALL_NOSPEC
> +		      : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +		        "+c" (control), "+d" (param1)
> +		      : "r" (__r8),

r8 is call-clobbered register, so you should use "+r" (__r8) to properly 
clobber it:

		        "+c" (control), "+d" (param1), "+r" (__r8)
		      : THUNK_TARGET(hv_hypercall_pg)

> +		      : "cc", "memory", "r9", "r10", "r11");
> +
> +	return hv_status;
> +}
> +#else
>   EXPORT_SYMBOL_GPL(hv_hypercall_pg);
> +#endif
>   
>   union hv_ghcb * __percpu *hv_ghcb_pg;
>   
> --- a/arch/x86/hyperv/ivm.c
> +++ b/arch/x86/hyperv/ivm.c
> @@ -376,6 +376,20 @@ int hv_snp_boot_ap(u32 cpu, unsigned lon
>   	return ret;
>   }
>   
> +u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
> +{
> +	u64 hv_status;
> +
> +	register u64 __r8 asm("r8") = param2;
> +	asm volatile("vmmcall"
> +		     : "=a" (hv_status), ASM_CALL_CONSTRAINT,
> +		       "+c" (control), "+d" (param1)
> +		     : "r" (__r8)

Also here:
		        "+c" (control), "+d" (param1), "+r" (__r8)
		      :

> +		     : "cc", "memory", "r9", "r10", "r11");
> +
> +	return hv_status;
> +}

Uros.