tech-privileged@lists.riscv.org | Quetion about xRET instruction

Oscar Jupp

Dear architect,

The spec section 3.3.2 said:

“An xRET instruction can be executed in privilege mode x or higher, where executing a lower-privilege xRET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack.”

It's not clear to me what is the application scenario for doing this?
Why execute a SRET instruction in M-mode?

Just change SIE, SPIE, SPP? Need to change privileged mode and other CSRs？

Regards,

Oscar Jupp

	jupposcar
jupposcar@...

Ved Shanbhogue

#1209

The ability to execute SRET in M-mode provides M-mode the
facility to trap, using TSR control, a SRET invoked in S-mode,
and after handling the trap to redo the SRET. The SRET invoked
in M-mode is not affected by the TSR control.

regards
ved

Show quoted text

On Fri, Dec 02, 2022 at 08:33:51PM +0800, Oscar Jupp wrote:
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       <span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Dear&nbsp;architect,&nbsp;</span></div><div><font face="Helvetica, Helvetica, 微软雅黑, 宋体">The spec section&nbsp;</font>3.3.2&nbsp;<span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">said:</span></div><div><span>“</span>An xRET instruction can be executed in privilege mode x or higher, where executing a lower-privilege xRET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack.”</div><div>It's not clear to me what is the application scenario for doing this?<br>Why execute a SRET instruction in M-mode?<span><br></span></div><div>Just change SIE, SPIE, SPP? Need to change privileged mode and other CSRs？<br></div><div><span><br></span></div><div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Regards,</div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Oscar Jupp</div></div>
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Allen Baum

#1210

I first read this as TSR meaning "Trap Service Routine",

but now I understand that it is the mstatus.TSR bit.

Show quoted text

On Fri, Dec 2, 2022 at 5:21 AM Ved Shanbhogue <ved@...> wrote:

The ability to execute SRET in M-mode provides M-mode the
facility to trap, using TSR control, a SRET invoked in S-mode,
and after handling the trap to redo the SRET. The SRET invoked
in M-mode is not affected by the TSR control.

regards
ved

On Fri, Dec 02, 2022 at 08:33:51PM +0800, Oscar Jupp wrote:
><html>
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><div style = 'font-family:Helvetica,Helvetica,微软雅黑, 宋体; line-height:1.6;'>
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> <br>
> </span>
> </div>
> <div>
> <span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Dear architect, </span></div><div><font face="Helvetica, Helvetica, 微软雅黑, 宋体">The spec section </font>3.3.2 <span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">said:</span></div><div><span>“</span>An xRET instruction can be executed in privilege mode x or higher, where executing a lower-privilege xRET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack.”</div><div>It's not clear to me what is the application scenario for doing this?<br>Why execute a SRET instruction in M-mode?<span><br></span></div><div>Just change SIE, SPIE, SPP? Need to change privileged mode and other CSRs？<br></div><div><span><br></span></div><div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Regards,</div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Oscar Jupp</div></div>
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Oscar Jupp

#1211

Dear architect,

Thank you for your reply.

I wonder how to emulate the hypervisor extension with trapping SRET on implementations that do not provide it.

Regards,

Oscar Jupp

	jupposcar
jupposcar@...

---- Replied Message ----

From	Ved Shanbhogue<ved@...>
Date	12/2/2022 21:21
To	Oscar Jupp<jupposcar@...>
Cc	tech-privileged@...<tech-privileged@...>
Subject	Re: [RISC-V] [tech-privileged] Quetion about xRET instruction

The ability to execute SRET in M-mode provides M-mode the
facility to trap, using TSR control, a SRET invoked in S-mode,
and after handling the trap to redo the SRET. The SRET invoked
in M-mode is not affected by the TSR control.

regards
ved

Show quoted text

On Fri, Dec 02, 2022 at 08:33:51PM +0800, Oscar Jupp wrote:

<html>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=UTF-8'>
</head>
<body>
<style>
font{
line-height: 1.6;
}
ul,ol{
padding-left: 20px;
list-style-position: inside;
}
</style>
<div style = 'font-family:Helvetica,Helvetica,微软雅黑, 宋体; line-height:1.6;'>
<div ></div><div>
<div>
<span>
<br>
</span>
</div>
<div>
<span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Dear architect, </span></div><div><font face="Helvetica, Helvetica, 微软雅黑, 宋体">The spec section </font>3.3.2 <span style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">said:</span></div><div><span>“</span>An xRET instruction can be executed in privilege mode x or higher, where executing a lower-privilege xRET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack.”</div><div>It's not clear to me what is the application scenario for doing this?<br>Why execute a SRET instruction in M-mode?<span><br></span></div><div>Just change SIE, SPIE, SPP? Need to change privileged mode and other CSRs？<br></div><div><span><br></span></div><div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Regards,</div><div style="font-family: Helvetica, Helvetica, 微软雅黑, 宋体;">Oscar Jupp</div></div>
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