linux-kernel - Re: [PATCH v9 3/8] x86/vmware: Introduce VMware hypercall API

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Date: Thu, 9 May 2024 16:42:00 -0700
From: Alexey Makhalov <alexey.makhalov@...adcom.com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
 hpa@...or.com, dave.hansen@...ux.intel.com, mingo@...hat.com,
 tglx@...utronix.de, x86@...nel.org, netdev@...r.kernel.org,
 richardcochran@...il.com, linux-input@...r.kernel.org,
 dmitry.torokhov@...il.com, zackr@...are.com,
 linux-graphics-maintainer@...are.com, pv-drivers@...are.com,
 timothym@...are.com, akaher@...are.com, dri-devel@...ts.freedesktop.org,
 daniel@...ll.ch, airlied@...il.com, tzimmermann@...e.de, mripard@...nel.org,
 maarten.lankhorst@...ux.intel.com, horms@...nel.org,
 kirill.shutemov@...ux.intel.com, Nadav Amit <nadav.amit@...il.com>,
 Jeff Sipek <jsipek@...are.com>
Subject: Re: [PATCH v9 3/8] x86/vmware: Introduce VMware hypercall API



On 5/7/24 2:58 AM, Borislav Petkov wrote:
> On Mon, May 06, 2024 at 02:53:00PM -0700, Alexey Makhalov wrote:
>> +#define VMWARE_HYPERCALL						\
>> +	ALTERNATIVE_3("cmpb $"						\
>> +			__stringify(CPUID_VMWARE_FEATURES_ECX_VMMCALL)	\
>> +			", %[mode]\n\t"					\
>> +		      "jg 2f\n\t"					\
>> +		      "je 1f\n\t"					\
>> +		      "movw %[port], %%dx\n\t"				\
>> +		      "inl (%%dx), %%eax\n\t"				\
>> +		      "jmp 3f\n\t"					\
>> +		      "1: vmmcall\n\t"					\
>> +		      "jmp 3f\n\t"					\
>> +		      "2: vmcall\n\t"					\
>> +		      "3:\n\t",						\
>> +		      "movw %[port], %%dx\n\t"				\
>> +		      "inl (%%dx), %%eax", X86_FEATURE_HYPERVISOR,	\
> 
> That's a bunch of insns and their size would inadvertently go into the final
> image.
> 
> What you should try to do is something like this:
> 
> ALTERNATIVE_3("jmp .Lend_legacy_call", "", X86_FEATURE_HYPERVISOR,
> 	      "vmcall; jmp .Lend_legacy_call", X86_FEATURE_VMCALL,
> 	      "vmmcall; jmp .Lend_legacy_call", X86_FEATURE_VMW_VMMCALL)
> 
> 		/* bunch of conditional branches and INs and V*MCALLs, etc go here */
> 
> 		.Lend_legacy_call:
> 
> so that you don't have these 26 bytes, as you say, of alternatives to patch but
> only the JMPs and the VM*CALLs.
> 
> See for an example the macros in arch/x86/entry/calling.h which simply jump
> over the code when not needed.
Good idea!

> 
> Also, you could restructure the alternative differently so that that bunch of
> insns call is completely out-of-line because all current machines support
> VM*CALL so you won't even need to patch. You only get to patch when running on
> some old rust and there you can just as well go completely out-of-line.
> 
Alternatives patching has not been performed at platform detection time.
And platform detection hypercalls should work on all machines.
That is the reason we have IN as a default hypercall behavior.

> Something along those lines, anyway.
> 
>> - * The high bandwidth in call. The low word of edx is presumed to have the
>> - * HB bit set.
>> + * High bandwidth calls are not supported on encrypted memory guests.
>> + * The caller should check cc_platform_has(CC_ATTR_MEM_ENCRYPT) and use
>> + * low bandwidth hypercall it memory encryption is set.
> 
> s/it/if/
Acked.

> 
>> -#define VMWARE_PORT(cmd, eax, ebx, ecx, edx)				\
>> -	__asm__("inl (%%dx), %%eax" :					\
>> -		"=a"(eax), "=c"(ecx), "=d"(edx), "=b"(ebx) :		\
>> -		"a"(VMWARE_HYPERVISOR_MAGIC),				\
>> -		"c"(VMWARE_CMD_##cmd),					\
>> -		"d"(VMWARE_HYPERVISOR_PORT), "b"(UINT_MAX) :		\
>> -		"memory")
>> -
>> -#define VMWARE_VMCALL(cmd, eax, ebx, ecx, edx)				\
>> -	__asm__("vmcall" :						\
>> -		"=a"(eax), "=c"(ecx), "=d"(edx), "=b"(ebx) :		\
>> -		"a"(VMWARE_HYPERVISOR_MAGIC),				\
>> -		"c"(VMWARE_CMD_##cmd),					\
>> -		"d"(0), "b"(UINT_MAX) :					\
>> -		"memory")
>> -
>> -#define VMWARE_VMMCALL(cmd, eax, ebx, ecx, edx)				\
>> -	__asm__("vmmcall" :						\
>> -		"=a"(eax), "=c"(ecx), "=d"(edx), "=b"(ebx) :		\
>> -		"a"(VMWARE_HYPERVISOR_MAGIC),				\
>> -		"c"(VMWARE_CMD_##cmd),					\
>> -		"d"(0), "b"(UINT_MAX) :					\
>> -		"memory")
>> -
>> -#define VMWARE_CMD(cmd, eax, ebx, ecx, edx) do {		\
>> -	switch (vmware_hypercall_mode) {			\
>> -	case CPUID_VMWARE_FEATURES_ECX_VMCALL:			\
>> -		VMWARE_VMCALL(cmd, eax, ebx, ecx, edx);		\
>> -		break;						\
>> -	case CPUID_VMWARE_FEATURES_ECX_VMMCALL:			\
>> -		VMWARE_VMMCALL(cmd, eax, ebx, ecx, edx);	\
>> -		break;						\
>> -	default:						\
>> -		VMWARE_PORT(cmd, eax, ebx, ecx, edx);		\
>> -		break;						\
>> -	}							\
>> -	} while (0)
> 
> You're kidding, right?
> 
> You went to all that trouble in patch 1 to move those to the header only to
> *remove* them here?
> 
> You do realize that that is a unnecessary churn for no good reason, right?
> 
> So that set needs to be restructured differently.
> 
> * first patch introduces those new API calls.
> 
> * follow-on patches convert the callers to the new API
> 
> * last patch removes the old API.
> 
> Ok?
My intention was to have a implementation transformation from locals 
macro through common macros to common API.

What you are suggesting will eliminate unnecessary patches. It makes sense.

Will perform this restructuring in v10.

> 
> And when you redo them, make sure you drop all Reviewed-by tags because the new
> versions are not reviewed anymore.
Noted.

Thanks again,
--Alexey