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Date: Tue, 7 May 2024 11:58:52 +0200
From: Borislav Petkov <bp@alien8.de>
To: Alexey Makhalov <alexey.makhalov@broadcom.com>
Cc: linux-kernel@vger.kernel.org, virtualization@lists.linux.dev,
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	Jeff Sipek <jsipek@vmware.com>
Subject: Re: [PATCH v9 3/8] x86/vmware: Introduce VMware hypercall API
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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.

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.

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/

> -#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?

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

Thx.


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette