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AJvYcCXRoiSq0Hp7rKDKTwur+CvvAw9abe2S3YBTSQNvjWxFD8o1G3jn58IybsL7Wc/oQT/6QQOlQFzz/4c=@lists.xenproject.org X-Gm-Message-State: AOJu0YykTVs4W1It4Pu7V/95XdDK01s+bDG5dI+DOF30HpfIVVL5fEBm R8Jxg0RdLse+MyeefzOibd65ljKlrZYkBgNAs82hDaNKK87E4lqCJgad3oKE6mo= X-Google-Smtp-Source: AGHT+IF5xmzxc7z9l4LODRF5nI3DelDJ3Lwq//KvTgatz+eV83ajEmG50ybkZr6BaYEwDYf9BzPO7w== X-Received: by 2002:a05:6512:1153:b0:52c:9906:fa33 with SMTP id 2adb3069b0e04-536ac32e23amr6676547e87.43.1727184654275; Tue, 24 Sep 2024 06:30:54 -0700 (PDT) Message-ID: Date: Tue, 24 Sep 2024 14:30:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/4] x86/boot: Refactor BIOS/PVH start From: Andrew Cooper To: Frediano Ziglio , xen-devel@lists.xenproject.org Cc: Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <20240924102811.86884-1-frediano.ziglio@cloud.com> <20240924102811.86884-3-frediano.ziglio@cloud.com> <91bf7854-83e8-42cc-a28b-21aaa472bb1f@citrix.com> Content-Language: en-GB Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 24/09/2024 2:25 pm, Andrew Cooper wrote: > On 24/09/2024 11:28 am, Frediano Ziglio wrote: >> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S >> index fa21024042..80bba6ff21 100644 >> --- a/xen/arch/x86/boot/head.S >> +++ b/xen/arch/x86/boot/head.S >> ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .long sym_offs(__pvh_start)) >> >> __pvh_start: >> - cld >> + mov $BOOT_TYPE_PVH, %dl >> + jmp .Lcommon_bios_pvh >> +#endif /* CONFIG_PVH_GUEST */ >> + >> +__start: >> + mov $BOOT_TYPE_BIOS, %dl > Even if we're generally using %dl, these must be full %edx writes. > > %edx commonly contains FMS on entry, and we don't want part of FMS left > in the upper half of the register. > >> + >> +.Lcommon_bios_pvh: >> cli >> + cld >> >> /* >> - * We need one call (i.e. push) to determine the load address. See >> - * __start for a discussion on how to do this safely using the PVH >> - * info structure. >> + * Multiboot (both 1 and 2) and PVH specify the stack pointer as >> + * undefined. This is unhelpful for relocatable images, where one >> + * call (i.e. push) is required to calculate the image's load address. >> + * >> + * Durig BIOS boot, there is one area of memory we know about with >> + * reasonable confidence that it isn't overlapped by Xen, and that's >> + * the Multiboot info structure in %ebx. Use it as a temporary stack. >> + * >> + * During PVH boot use info structure in %ebx. >> */ >> >> /* Preserve the field we're about to clobber. */ >> - mov (%ebx), %edx >> + mov (%ebx), %ecx > Both here, and ... > >> lea 4(%ebx), %esp >> >> /* Calculate the load base address. */ >> @@ -449,62 +459,40 @@ __pvh_start: >> mov %ecx, %es >> mov %ecx, %ss >> >> - /* Skip bootloader setup and bios setup, go straight to trampoline */ >> - movb $1, sym_esi(pvh_boot) >> - movb $1, sym_esi(skip_realmode) >> - >> - /* Set trampoline_phys to use mfn 1 to avoid having a mapping at VA 0 */ >> - movw $0x1000, sym_esi(trampoline_phys) >> - mov (%ebx), %eax /* mov $XEN_HVM_START_MAGIC_VALUE, %eax */ >> - jmp trampoline_setup >> - >> -#endif /* CONFIG_PVH_GUEST */ >> + /* Load null selector to unused segment registers. */ >> + xor %ecx, %ecx >> + mov %ecx, %fs >> + mov %ecx, %gs >> >> -.Linitialise_bss: >> /* Initialise the BSS. */ >> - mov %eax, %edx >> - >> + mov %eax, %ebp > ... here, we've got changes caused by now using %edx for a long-lived > purpose (and a change in linebreaks.) > > For this, %ebp should be used straight away in patch 1.  I've not > committed it yet, so can fix that up. > > > I have to admit that I think this patch would be easier if the "use %ebx > for BOOT_TYPE_*" change was split out of "better merge the BIOS/PVH > paths".  That would at least get the incidental %edx changes out of the way. > > Also, inserting > > #ifdef CONFIG_PVH_GUEST >         cmp     $BOOT_TYPE_PVH, %dl >         jne     1f > 1: > #endif /* CONFIG_PVH_GUEST */ > > in the same patch will probably make the subsequent diff far more legible. > > Thoughts? > > I might give this a quick go, and see how it ends up looking... Actually, why do we need BOOT_TYPE_* at all?  We've already got BOOTLOADER_MAGIC in %eax which can be used to identify PVH. ~Andrew