On Thu, May 11, 2017 at 11:22:02PM +0000, Xu, Anthony wrote:
SeaBIOS has a couple of different methods to accomplish this mode switching - it can directly switch modes (C16_BIG switch) or it can use a helper in SMM mode to perform the switch (C16_SMM). The preferred method is C16_SMM as C16_BIG isn't available in VM86 mode and it also clobbers the 16bit segment registers.
Hi Kevin,
Why does SeaBios use VM86 mode?
As Paolo mentioned, SeaBIOS doesn't use it, but it must support callers that use it.
vtx supports real mode long time ago.
C16_SMM is slow because it causes several extra switches between VM and QEMU. I would prefer C16_BIG for better performance. Does SeaBios know if it runs in real mode or VM86 mode? Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?
Yes, SeaBIOS could use the smsw instruction (see x86.h:cr0_vm86_read) to check if it is in VM86 mode. Using C16_BIG would still clobber the segment registers, but it might be worth that risk to avoid the extra overhead of C16_SMM.
-Kevin
On 12/05/2017 18:26, Kevin O'Connor wrote:
On Thu, May 11, 2017 at 11:22:02PM +0000, Xu, Anthony wrote:
SeaBIOS has a couple of different methods to accomplish this mode switching - it can directly switch modes (C16_BIG switch) or it can use a helper in SMM mode to perform the switch (C16_SMM). The preferred method is C16_SMM as C16_BIG isn't available in VM86 mode and it also clobbers the 16bit segment registers.
Hi Kevin,
Why does SeaBios use VM86 mode?
As Paolo mentioned, SeaBIOS doesn't use it, but it must support callers that use it.
vtx supports real mode long time ago.
C16_SMM is slow because it causes several extra switches between VM and QEMU. I would prefer C16_BIG for better performance. Does SeaBios know if it runs in real mode or VM86 mode? Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?
Yes, SeaBIOS could use the smsw instruction (see x86.h:cr0_vm86_read) to check if it is in VM86 mode. Using C16_BIG would still clobber the segment registers, but it might be worth that risk to avoid the extra overhead of C16_SMM.
I don't think the SMM overhead is that bad (is SMM only used on QEMU?). At least for KVM it's not bad, maybe 4000 clock cycles roundtrip (SeaBIOS does two such trips).
For TCG I have a patch to reduce it substantially but I haven't had time to send it yet.
Paolo
On Fri, May 12, 2017 at 06:29:19PM +0200, Paolo Bonzini wrote:
On 12/05/2017 18:26, Kevin O'Connor wrote:
On Thu, May 11, 2017 at 11:22:02PM +0000, Xu, Anthony wrote:
SeaBIOS has a couple of different methods to accomplish this mode switching - it can directly switch modes (C16_BIG switch) or it can use a helper in SMM mode to perform the switch (C16_SMM). The preferred method is C16_SMM as C16_BIG isn't available in VM86 mode and it also clobbers the 16bit segment registers.
Hi Kevin,
Why does SeaBios use VM86 mode?
As Paolo mentioned, SeaBIOS doesn't use it, but it must support callers that use it.
vtx supports real mode long time ago.
C16_SMM is slow because it causes several extra switches between VM and QEMU. I would prefer C16_BIG for better performance. Does SeaBios know if it runs in real mode or VM86 mode? Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?
Yes, SeaBIOS could use the smsw instruction (see x86.h:cr0_vm86_read) to check if it is in VM86 mode. Using C16_BIG would still clobber the segment registers, but it might be worth that risk to avoid the extra overhead of C16_SMM.
I don't think the SMM overhead is that bad (is SMM only used on QEMU?).
Yes, the SMM code in SeaBIOS is currently exclusive to QEMU. I considered adding an equivalent SMI handler to coreboot, but it seems it's not really needed. Only ancient OSes require it and that's not a typical mix with coreboot. (And if one needs to run an ancient OS then they would almost certainly be better off running it in an emulator anyway).
At least for KVM it's not bad, maybe 4000 clock cycles roundtrip (SeaBIOS does two such trips).
For TCG I have a patch to reduce it substantially but I haven't had time to send it yet.
-Kevin
-----Original Message----- From: Paolo Bonzini [mailto:pbonzini@redhat.com] Sent: Friday, May 12, 2017 9:29 AM To: Kevin O'Connor kevin@koconnor.net; Xu, Anthony anthony.xu@intel.com Cc: seabios@seabios.org; kraxel@redhat.com; stefanha@gmail.com; Zhong, Yang yang.zhong@intel.com Subject: Re: [SeaBIOS] A20: ahci_process_op is called with A20 off
On 12/05/2017 18:26, Kevin O'Connor wrote:
On Thu, May 11, 2017 at 11:22:02PM +0000, Xu, Anthony wrote:
SeaBIOS has a couple of different methods to accomplish this mode switching - it can directly switch modes (C16_BIG switch) or it can use a helper in SMM mode to perform the switch (C16_SMM). The preferred method is C16_SMM as C16_BIG isn't available in VM86 mode and it also clobbers the 16bit segment registers.
Hi Kevin,
Why does SeaBios use VM86 mode?
As Paolo mentioned, SeaBIOS doesn't use it, but it must support callers that use it.
vtx supports real mode long time ago.
C16_SMM is slow because it causes several extra switches between VM
and QEMU.
I would prefer C16_BIG for better performance. Does SeaBios know if it runs in real mode or VM86 mode? Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?
Yes, SeaBIOS could use the smsw instruction (see x86.h:cr0_vm86_read) to check if it is in VM86 mode. Using C16_BIG would still clobber the segment registers, but it might be worth that risk to avoid the extra overhead of C16_SMM.
I don't think the SMM overhead is that bad (is SMM only used on QEMU?). At least for KVM it's not bad, maybe 4000 clock cycles roundtrip (SeaBIOS does two such trips).
For each trip, there are two switches between SeaBios and QEMU, One is caused by outb %%al, $" __stringify(PORT_SMI_CMD) to enter SMM mode
The other is caused by u8 cmd = inb(PORT_SMI_CMD) in smm handler
Anthony
For TCG I have a patch to reduce it substantially but I haven't had time to send it yet.
Paolo
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Kevin O'Connor -
Paolo Bonzini -
Xu, Anthony